So what you want is a "giant Behemoth".
In my long answer I discuss modifying the planet of your story, if it is not Earth, to make large creatures more probable, discuss the size records of known creatures on Earth, and discuss how to modify your creatures to make them more mammoth.
Part One of Ten: Being A Pedant.
I'm working on a species of fantasy creature based OFF the mythological Behemoth
I believe that correct grammar is to say "based OFF OF the mythological Behemoth" or "based ON the mythological Behemoth". And the second sounds better to me.
Part two of Ten: Making The Planet Suitable.
Planetary characteristics to consider include both the surface gravity and the escape velocity. The surface gravity determines how much creatures weigh and how strong their bones and muscles have to be. The escape velocity determines the planet's ability to retain an atmosphere. And of course the type of atmosphere a planet has depends on both what atmosphere it produces or acquires and its ability to retain an atmosphere.
And I have noticed that surface gravity and escape velocity do not change equally. I have noticed that for planets and other bodies in the solar system with less mass than Earth their escape velocities and surface gravity do not have the same ratio compared to Earth's escape velocity and surface gravity. The escape velocity of a smaller world will be slightly larger, relative to Earth's, than the surface gravity.
This suggests to me that a planet with less mass than Earth should be a good setting for your giant behemoths.
How much can you change the mass of your fictional planet while still having a breathable atmosphere?
You should read Habitable planets for Man, Stephen H. Dole, 1964, 2007, which has a section on the size range of planets habitable for humans, and thus for other large land based vertebrates.
Part Three of Ten: A highly Oblate Planet?
Another factor to consider is making your planet rotate very fast and have an oblate shape. Surface gravity will be higher at the poles and lower at lower latitudes, being the lowest at the equator, where your giant behemoths might live.
The classic example of a highly oblate planet in science fiction is Mesklin in Mission of gravity by Hal Clement (1953). Mesklin is very large and rotates very fast, so the surface gravity is three g at the equator and hundreds of g at the poles.
I am not sure about whether the atmosphere would have uniform density or be denser at the poles. I also wonder what the escape velocity would be at the equator of a rapidly spinning world and whether the atmosphere would escape from the equator.
Some people speculate that a large moon, like the Moon, is necessary for a planet to have large tides and since Earth life might have began in tidal zones, necessary for a planet to have life. Since the Moon has been receding and slowing down Earth's spin for billions of years before complex life appeared, an initially highly oblate planet might slow down its spin and loose most of its oblateness before it developed an oxygen rich atmosphere and large lifeforms.
But maybe advanced aliens came to a rapidly spinning new planet and terraformed it with an oxygen rich atmosphere and seeded it with advanced lifeforms from their planet, and over millions of years the giant behemoths evolved on that planet while it was still spinning rapidly.
Or maybe a large moon is not necessary for life on a planet.
Part Four: A Jinxed Planet?
Another interesting science fiction world is Jinx in Larry Niven's Known Space series.
Jinx, orbiting Sirius A, is a massive moon of a gas giant (called Primary), stretched by tidal forces into an egg shape and tidally locked. In the habitable areas it has high surface gravity near the limits of human extended tolerance. The points nearest to and farthest from Primary (called the "East" and "West" ends) lie elevated out of the atmosphere in vacuum. The atmosphere of the belt-region halfway between them is too dense and too hot to breathe, and is inhabited only by the Bandersnatchi. The zones between the vacuum areas and the high-density belt area have atmosphere breathable by humans. Jinx's "East" and "West" ends become a major in vacuo manufacturing area. Jinxian humans are short and squat, the strongest bipeds in Known Space. But they tend to die early, from heart and circulatory problems. There is a tourist industry which provides substantial useful interplanetary trade credits for the Bandersnatchi, who allow themselves to be hunted by humans under strict protocols.
So a Jinx-like world might have the right surface gravity and atmosphere in some regions for giant behemoths to flourish.
Part Five of Ten: The Largest Known Dinosaurs.
The largest terrestrial animal known to history is the Argentinasaurus, which by some estimates weighed up to 100 tons. (Meanwhile, Elephants weight around just 6). Some have said this is the absolute size limit for terrestrial land animals.
If so, what changes would need to be made to make an organism's design exceed that mass? Like, 200-500 tons?
The weight of the largest dinosaurs is not estimated with great certainty.
For example, Wikipedia has list of the heaviest sauropodmorphs:
Argentinosaurus huinculensis: 50–100 t (55–110 short tons)
Patagotitan mayorum: 55–77 t (61–85 short tons)
Mamenchisaurus sinocanadorum: 50–80 t (55–88 short tons)
Notocolossus gonzalezparejasi: 44.9–75.9 t (49.5–83.7 short tons)
Alamosaurus sanjuanensis: 39.5–73 t (43.5–80.5 short tons)
Apatosaurus ajax: 32.7–72.6 t (36.0–80.0 short tons)
Sauroposeidon proteles: 40–60 t (44–66 short tons)
Dreadnoughtus schrani: 22.1–59.3 t (24.4–65.4 short tons)
Paralititan stromeri: 20–59 t (22–65 short tons)
Many people would disagree with the relative order of the various dinosaurs and with the weight estimates for various species. And of course new large sauropodmorphs are discovered all the time and such lists become more and more obsolete as time passes.
Part Six of Ten: The Largest Not Well Known Dinosaurs?
And of course that list omits the two most controversial and possibly largest sauropod dinosaurs ever discovered, known from now lost fossils.
There was Amphicoelias fragillimus, now reclassified as Maraapunisaurus fragillimus, which might possibly according to some interpretations been up to 60 meters (200 feet) long and weighed up to 150 tonnes (150 long tons, 170 short tons). Some estimates make it much smaller.
The third named Amphicoelias species, A. fragillimus, was known only from a single, incomplete 1.5 m (4.9 ft) tall neural arch, either last or second to last in the series of back vertebrae. Based only on an illustration published in 1878, this vertebra would have measured 2.7 meters (8.9 ft) tall in life. However, it has been argued that the scale bar in the published description contained a typographical error, and the fossil vertebra was in fact only 1.38 meters (4.5 ft) tall. In addition to this vertebra, Cope's field notes contain an entry for an "[i]mmense distal end of femur”, located only a few tens of meters away from the giant vertebra. It is likely that this undescribed leg bone belonged to the same individual animal as the neural spine, but it was never collected or described. In 2018, A. fragillimus was given its own genus, Maraapunisaurus, and reclassified as a primitive rebbachisaurid.
While M. fragillimus as a sauropod would be relatively elongated, its enormous size still made it very massive. Weight is much more difficult to determine than length in sauropods, as the more complex equations needed are prone to greater margins of error based on smaller variations in the overall proportions of the animal. Carpenter in 2006 used Paul's 1994 estimate of the mass of Diplodocus carnegii (11.5 metric tons (11.3 long tons; 12.7 short tons)) to speculate that M. fragillimus could have weighed up to 122.4 metric tons (120.5 long tons; 134.9 short tons). The heaviest blue whale on record weighed 173 metric tons (170 long tons; 191 short tons), and the heaviest dinosaur known from reasonably good remains, Argentinosaurus, weighed 80 to 100 metric tons (79 to 98 long tons; 88 to 110 short tons), although if the size estimates can be validated, it could still be lighter than Bruhathkayosaurus, which has been estimated to have weighed 126 metric tons (124 long tons; 139 short tons), but is also known from highly fragmentary remains. In 2019 Gregory S. Paul estimated Maraapunisaurus at 35-40 meters (115-131 feet) in length and 80-120 tonnes (88-132 short tons) in weight with a femoral length of 3-3.5 meters (10-11.5 ft) or more, larger than Carpenter's estimation .
Possibly the most controversial sauropod fossil of all was Bruhathkayosaurus.
Bruhathkayosaurus (/bruːˌhæθkeɪoʊˈsɔːrəs/; meaning "huge-bodied lizard") is a genus of dinosaur found in the Kallemedu Formation of India. The fragmentary remains were originally described as a theropod but later publications listed it as a sauropod. Estimates by researchers exceed those of the titanosaur Argentinosaurus,2 as longer than 35 metres (115 ft) and weighing over 80 tons. All the estimates are based on the dimensions of the fossils described in Yadagiri and Ayyasami's 1987 paper, which announced the find. In 2017 it was reported that the original fossils had disintegrated and no longer exist.
No total body size estimates for Bruhathkayosaurus have been published, but paleontologists and researchers have posted tentative estimates on the Internet. In a post from June 2001, Mickey Mortimer estimated that Bruhathkayosaurus could have reached 40–44 m (131–144 ft) in length and might have weighed 175–220 tons, but in later posts retracted these estimates, reducing the estimated length of Bruhathkayosaurus to 28–34 m (92–112 ft), and declined to provide a new weight estimate, describing the older weight estimates as inaccurate. In a May 2008 article for the weblog Sauropod Vertebra Picture of the Week, paleontologist Matt Wedel used a comparison with Argentinosaurus and calculated the weight of Bruhathkayosaurus at up to 126 metric tons (139 short tons). In 2019 Paul suggested that the supposed tibia is probably a degraded femur, in which case its length was slightly greater than that of Dreadnoughtus (1.91 meters) and Futalognkosaurus (1.98 meters). So he estimated its mass between 30-55 tonnes (33-61 short tons) much lower than any previous estimation.
So some tentative and probably inaccurate mass estimates of Bruhathkayosaurus put the upper limit of the mass range a little bit above the lower limit of your goal of 200 to 500 tons.
Part Seven of Ten: The Largest Living Land Mammals.
As for elephants, baby elephants are in the human mass range, while adult females average less massive than adult males.
The smallest living elephant species is the African forest elephant Loxodonta cyclotis.
Bulls reach a shoulder height of 2.4–3.0 m (7.9–9.8 ft). Females are smaller at about 1.8–2.4 m (5.9–7.9 ft) tall at the shoulder. They reach a weight of 2–4 tonnes (2.2–4.4 short tons). Foot print size ranges from 12.5 to 35.3 cm (4.9 to 13.9 in).
The middle sized elephant species is the Asian elephant Elephas maximus.
On average, males are about 2.75 m (9.0 ft) tall at the shoulder and 4 t (4.4 short tons) in weight, while females are smaller at about 2.4 m (7.9 ft) at the shoulder and 2.7 t (3.0 short tons) in weight. Length of body and head including trunk is 5.5–6.5 m (18–21 ft) with the tail being 1.2–1.5 m (3.9–4.9 ft) long. The largest bull elephant ever recorded was shot by the Maharajah of Susang in the Garo Hills of Assam, India in 1924, it weighed an estimated 7 t (7.7 short tons), stood 3.43 m (11.3 ft) tall at the shoulder and was 8.06 m (26.4 ft) long from head to tail. There are reports of larger individuals as tall as 3.7 m (12 ft).
The largest living elephant species is the African bush or savanna elephant, Loxodonta africana.
The African bush elephant is the largest and heaviest land animal on Earth, being up to 3.96 m (13.0 ft) tall at the shoulder and an estimated weight of up to 10.4 t (11.5 short tons). On average, males are about 3.20 m (10.5 ft) tall at the shoulder and weigh 6.00 t (6.61 short tons), while females are much smaller at about 2.60 m (8.53 ft) tall at the shoulder and 3.00 t (3.31 short tons) in weight. Elephants attain their maximum stature when they complete the fusion of long-bone epiphyses, occurring in males around the age of 40 and females around the age of 25.
Since the average mass of male African bush elephants is about the six tons you mentioned for elephants, it is not surprising that many males are more massive, some much more.
Part Eight of Ten: The Largest Extinct Land Mammals:
The largest land mammal extant today is the African bush elephant. The largest extinct land mammal known was long considered to be Paraceratherium orgosensis, a rhinoceros relative thought to have stood up to 4.8 m (15.7 ft) tall, measured over 7.4 m (24.3 ft) long and may have weighed about 17 tonnes. In 2015, a study suggested that one example of the proboscidean Palaeoloxodon namadicus may have been the largest land mammal ever, based on extensive research of fragmentary leg bone fossils from one individual, with a maximum estimated size of 22 tonnes.
So known extinct land mammals may have weighed two, three, or four times as much as your 6 ton elephants.
Part Nine of Ten: Vertebrates With More Limbs.
One way to make giant behemoths larger would be to give them more limbs.
the Op asks for a vertebrate body plan, not a tetrapod body plan. So maybe the giant behemoths could be vertebrate hexapods, octopods, decapods, etc. Their torsos might have the same spacing between pairs of legs as giant sauropod dinosaurs had, but with more pairs of legs, thus doubling or more the possible mass of the torso.
If more than four limbs are permissible, the giant behemoths could even have tens or hundreds of pairs of legs and be built like vertebrate caterpillars, centipedes, or millipedes, with a section of torso between each pair of legs with the mass of an elephant or brontosaurus torso.
And possibly such a multi legged creature could be like a giant hippo or crocodile, mostly living in lakes, rivers, or oceans, but sometimes walking on land, and mostly feeding on plants but sometimes getting vital nutrients by eating smaller creatures.
Part Ten of Ten: Smaller Creatures Linked Together To Make A Larger One:
If that is not permissible, possible the species could be contain individuals the sizes of elephants or sauropod dinosaurs, who occasionally join with other individuals to form a hive creature, a really giant behemoth.
Such a larger creature formed by the linking of smaller creatures was in "The Planet of Doubt" by Stanley G. Weinbaum, Astounding Stories October, 1935. The linked creatures were based on actual behavior of a species on Earth.
...When they receive a signal from the ship, they hurry back to find it under attack by an immensely long black creature made up of dozens of connected segments. They manage to fight it off and return to the ship. Burlingame decides that the creature is similar to the larval Thaumetopoeidae, which forms processions when it travels from its nest. She hypothesizes that the individual segments link nervous systems so that they all act in unison. As for the shapes in the fog, Burlingame thinks they are analogous to honeyguides, and that they lead the segment-creature to its prey.
I think that the linking of nervous systems is not found in any Earth animals, but would tend to make the linked super behemoth more or an individual instead of a herd.
And possibly when the creatures link up they might pass partially digested food from the anus of one creatures to the mouth of the one behind it, which would extract some nutrients and pass it on to the creature behind it. That sounds disgusting to me, but it would make them a bit more like a creature instead of a herd.
And it seems to me that as many behemoths as could live in the area as a herd of separate behemoths could link together from time to time to form a more or less hive giant behemoth.