They are viable, even on earth without all those boosts you imagine.
some blimp pros for earth
The energy efficiency of blimps is on pair with railroads, it takes 3rd place this way - sea ships(0.3), railroads(1x), blimps(2x of railroads energy-wise), trucks (4x), airplanes(10-30x). Comparisons are very approximate and rely on that I remember the number for blimps correctly and didn't screw my calculations back then and that blimps move quite slow(20-40kph).
Blimps are one and pretty much only one which can use high(8-20km) altitude Jet streams having some "free" energy for long-distance hauling. Its next competitors are solar-powered airplanes, but those aren't that impressive fragile and have other problems including complexity and EROEI.
In the capacity to lift and deliver bulky cargo - blimps are the only ones that can deliver something like oil platforms from the assembly point to installation point in one go(ignore the fact it is built in the same places where ships are, it is just an example of capacity and size). And not only those are a problem, and it could be an improvement if we had such capacity. At the moment it is - or a complicated transporting problem, or requires all kinds of moves from design to installation procedures.
Blimps are the only cargo transport that can cover 100% of the planet's surface.
Blimps do not require loading unloading at coast ports and can deliver directly to a consumer, faster than a sea ship, with less energy spent than a ship(if we rely on jet streams, okay okay I hear you solar panels, not necessarily a great idea but "free" energy anyway) and if we consider that from a port this cargo has to be delivered further by trucks (often) or railroads(also often) then energy efficiency can be even higher and maybe even sufficient to compete with railroads or maybe more(no loading unloading procedures, delays, etc).
If one needs a port for cargo for blimps then such port can be built anywhere, you can choose an optimal place for that type of cargo. Seaport, for bulk cargo transporting, is a huge system - with roads, railroads attached to it, with places to store cargo, load-unload and places for it are limited to the coastline - we did cope with that but it does not mean things could not be better.
In a time when we begin to see bulk gas transporting - blimps can be used for that, and it hard to tell who's the winner - ship or a blimp, so besides bulk cargo transporting it another one where blimps can compete with sea ships transporting directly, again cutting corners (literally) by not being bound by channels, and deliver directly to gas storages, directly from gas "wells" meaning cutting pipes, deliver faster(2-3x) than doing so by ships, be flexible and more responsive for demand changes. The pipe is a winner for fixed places hands down, but there is not always a pipe, and one can test and build up production to a level when a pipe investment makes sense - so more flexibility and blimps rather win over ships.
- the probably least useful case for the op, with modular reactors and such, as the same reactors can be used for what most of that gas is used for.
City to city transportation can be more efficient - essentially flexibility of a helicopter for a price(energy-wise, delivery) 0.5x of a truck, with direct routes, higher loads(more than 40t), more efficient lower loads(0.1-1-5t), better prospects of making drones out of those, etc - those can be attractive points.
op's materials, energy source
The importance of different points changes slightly, due to reactors' use, but overall it is the same. Gravity and density of atmosphere have minimal influence on all those points. Stronger cheaper materials are beneficial, but so are they for other things, I mean they are not a game-changing factor and not required but sure they are a good thing.
Given that nuclear reactors don't work for heavier-than-air flight, lighter-than-air flight seems even less plausible. The high surface area of lighter-than-air vehicles does make solar a more viable option, though. – @jdunlop
This one is wrong. Reactors definitely are more plausible for blimps. There are few reasons for that - liftoff energy consumption may be low percents of full throttle, while on airplanes it is one of the intense moments. Not necessarily a big deal for combustion engines, but reactors do not like jumps in power for many reasons, including for reasons of processes of chain reactions happening in nuclear fuel. The difference is not necessarily huge, but a similar difference is enough to not have turbines on bikes and cars(it is not only it but one of them).
Power to weight ratio is less important for a blimp, the range is wider - if a blimp can lift one, then it does not matter that much if it delivers 1MW or 50MW - the difference is cruise speed, but not the ability to fly if the airplane does not have sufficient power it just can't take off, even if this power would be sufficient to hold some cruise speed.
Solar panels for airplanes and blimps are not necessarily such a cool thing - there are airplanes like those so it can be seen(quite fragile construction to which op's materials could be a great help btw), for blimps it may be a better option than for airplanes, but it hard to tell if it better than some jet stream sail as an example, which can work 24.7 without batteries.
In an airplane, one can't distance himself from a reactor, while on a blimp there is more distance between the crew and a reactor, which may mean lesser shielding requirements(thus mass, etc). A blimp can tow a blimp, from start to finish, essentially making an external engine, this is used for ships, but airplanes, it is used for gliders but beyond that, it is even harder to imagine a Boeing size thing towing another Boeing - but a blimp towing another blimp, not a big deal.
More airplanes did crush than blimps burned - (anonymous)
Safety-wise it is hard to argue what's better, essentially we are noobs blimps making and did perfect designs for ships and airplanes for more than a hundred years. And despite that there are spectacular design failures for ships in history('70s, 80's, in essence, worse than titanic in terms of mass/cargo loss), so as cruise ships incidents in this century. Airplanes are not an exception, we all know that, airplanes do have some problems landing as well and it is hard to imagine it going away any time soon(it can, with better technologies, like smart matter but when it happens is unknown)
Better materials make blimps more useful already, and some are in limited use for inspecting and such as a replacement for ground vehicles and helicopters. This is far from potential it has, but it is, even more, a sign that blimps can offer unique advantages. So as for safety materials are an improvement and not only materials but other technologies like detecting gases - nothing of that was possible back in the days but still, some blimps of that era had million+ km's under their belt.
They tend to descend than to crash is definitely more friendly for reactor placement, not as good as tends to sink in water, but still.
Mooring lines - what about a 3km landing strip, eh?. Airplanes usually do not idle in hangars they fly in and out as much as they can - so storing is overrated.
Autopilot for airplanes works well enough, take-off and landing are still a procedure but great improvement here as well, for blimps drone-like capacities are even easier and that (combined with Starlink) can offer unprecedented opportunities, and if you still care about storing problems - here as well.
So in general with or without reactors and more new materials blimps look like an attractive idea, there are interconnected factors why it does not happen as of today, including we did bend our processes to not need them, but it does not mean they have no chance for resurrection, and for places where their development wasn't abandoned and they continue to evolve and processes were shaped for their use - most likely our system would be unimaginable and not viable.
If someone can beat sea transporting then they will have all the chances to spread that technology to other places - they will have technologies and money for that as SpaceX did. And one of the avenues is transporting gas overseas and from small places.