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Ballistas could be given the "Mountain Guard" ability, preventing them from being moved from their initial position. This would stop them from moving, and it would be impossible to move them. In this case, a movement animation would definitely not be needed.
If, however, you want them to not be blocked by everything, you could make the cannon fire non-blocking by implementing it as an exploit, i.e., give them a skill that is not dependent on the presence of an enemy nearby, for example, a modified version of Jarl's Flame Strike or Titan's Lightning. Then, it could fire even if an enemy unit is standing right next to it (the damage could be reduced compared to the main fire, or it could be made the only possible firing option; this is up to you, and the amount of ammunition can be regulated by the number of times the skill is used, or better yet, made infinite, since they cannot move). Profit.
If you go another way, namely, give them the ability to move, can you attach the position of one of the dwarves and the ballista to the position of another moving dwarf with an effect, or will it pop out beyond a certain area of the model? Or at least just the ballista to one of the dwarves. Then it will move in sync with him, and during this process, attach wheels to the ballista itself with an effect, say, based on a water wheel (if it is possible to attach animations from the game engine, of course), so that they rotate at least minimally.
By the way, replacing the ballista with a cannon or any other weapon from the suggestions in the comments above is very logical, even based on world history. For example, if your army uses bolt-action rifles, what will you do when you invent a semi-automatic action? That's right, you'll replace the rifles with semi-automatic ones. And if you invent automatic weapons? That's right, and so on. The same goes for if you used to use bows and arrows, but then firearms appeared, and suddenly chivalry died out; there are many examples. Therefore, you can make their building some kind of workshop where advanced siege weapons are developed, or this weapon is already known, and the upgrade of the workshop is like its re-equipment, necessary for the development of more complex and effective combat mechanisms. The units are still the same - artillery engineers, or whatever you call them, but the combat machines are different. Although at least one upgrade should develop and expand the mechanics of the basic unit, that's for sure