Ever seen a baleen whale hunt? Once they find their prey, they circle it from below and blow bubbles. The bubbles form a wall which keeps the fish trapped, so the whale can snatch them.
Your dragon can do the same thing. Enclose the sheep in circles of fire. Any sheep that hides in a hole should also be torched, to discourage that.
Maximum size for flying vertebrates on Earth
Quetzalcoatlus reached 250kg with a wingspan of 10.4m. This is your best basis to work from. This paper provides a Relative Failure Force for Quetzalcoatlus' humerus of 1.85 (that is 1.85x it's body mass). This should equate to around 450kg (without accounting for any scaling) as a theoretical biomechanical limit, ...
There are problems
You can't be a cold blooded vertebrate flyer, you can't maintain enough warmth in flight to get anywhere without a high metabolism. wings are too effective at cooling. Also flight is high energy demand, so that s another reason you will not see it in a cold blooded animal (well not one that weights more than a few ounces). Also feathers ...
Your wyverns are giant hoatzins.
Using a neotenous hoatzin gets past the hardest part of your requirements: a flying quadruped. Hoatzin chicks have claws on their wings and use them with their legs to walk like a lizard.
In a tank of ...
Whenever you have the option, use weapons
Weapons are force multipliers and equalizers. An elementary schooler has almost no chance in an unarmed fight with an adult. With swords or knives they're going to still almost certainly lose, but there's a chance of them doing some real damage and possibly getting in a lucky stab that decides the fight, and a child ...
Scaring them out of the depressions
This will mostly help for the depressions part. Lots of sheep and such actually live in woodlands, which is much more difficult.
Have the dragon pick up a boulder. Doesn't need to be huge either, as a decent sized stone can already hurt the prey.
Now fly over and drop. The stone will hit a creature or closeby enough to ...
One reason could be the choice to live one's life on a different time scale. Contemplating other life forms or planetary evolution over multiple generations, or at least very long time periods, the time of a single tree generation, may be sufficient motivation.
Treelike people could gather 1000 years of data (geologic, entomologic, climatic,....
Not necessary: because a cloaca is mostly hidden by scales. However if your story requires the use of some type of clothing, think along the lines of a Chinese finger trap: something that in normal use would tightly cling to the body, but by removing in the opposite direction of slithering would be generally easy.
Characteristics of harbor seals, that could easy be adapted into human behavior:
they eat a lots of fish (grown ups eat only fish, young ones eat other sea animals like crabs too)
they do not like company, they only meet each other on the basking area and for mating
they do not like to being touched by other harbor seals (for example to sun bath: there ...
Movement costs too much energy.
There is a good reason plants and trees still exist. Even without being the start of a lot of food chains, it has many advantages. Moving costs an insane amount of energy. Not moving at the right spot will allow you to thrive with using a fraction of the energy. Not using energy is a huge advantage, as you'll have less to ...
You don't mention if your creature has upper limbs or not.
If it has no upper limbs, the only option seems to be to have a sort of sleeve with something clingy on their body, either by using a belt in an area not in contact with the ground or by using elastic fabric that fits snug.
If they have upper limbs, on top of the solutions mentioned above they can ...
maybe for camouflage? sound like your creature live inside forest at least, there some insect that look like plant for example like this
as illegal logging say, to hide the logs, hide it inside the forest.
Life Span and Size
Trees (depending on type) can be among the largest and longest-lived lifeforms on the planet.
If your beings value these traits higher than mobility (or maybe your tree-like beings are mobile) then a tree is a good choice for them.
Regarding warfare, the essential parts is about team work and coordination.
I suggest to use both Spear-Fu and Shield-Fu in regards to warfare scenario for both races, at least it's cheap, which probably what goblins can afford. If the goblins can afford better, I suggest Montante-Fu or Zweihander-Fu for the goblins which is more fitting for your scenario. ...