Why not just allow the AI to attack forts? Bombing forts lowers their effectiveness. Fortress busters could get some actual use.
I have no idea how to force AI to attack high level forts.
In connection with this situation, another player tested the AI behavior in my Mod in relation to high level forts, just for interest.
It turned out that the AI was happy to attack 5-6 level forts. And AI does not want to attack 7-8-9-10 levels forts.
I guess probably if I would reduce the penalty of the 10 level fort from the current -80 to -70/-60, then this will probably allow the AI to attack the forts of 7/8 levels.
But even such edit will not change the ban on attacking a 10 level fort.