It's also possible to weaken the ballista in regular settings. I'm only interested in changing the linearity of the damage increase or its dependence on something else.

Yes, Dyrman is right, there's no point in weakening the ballista, but adding damage to it would be extremely unbalanced. It would be more reasonable to add hit points to the ballista when the "War Machines" skill is increased. That is, at the basic level of the "War Machines" skill, everything would remain unchanged, but at the advanced level, you could add, for example, 200 hit points, and at the expert level, increase it by 400 hit points. This way, it would be more difficult to destroy the ballista in one hit.

The catapult is a rare occurrence, so it doesn't need to be taken into account, at least not here.

Perhaps it's rare, but each of us has to attack enemies and capture castles sooner or later, and sometimes even defend our own. In the case of the catapult, you could also increase its hit points when the skill is developed.

Added 6 minutes later
However, I'm not the author of the mod, so I can't impose my opinion - I can only recommend it. The final decision is up to the author.