I think I got it worked out, somewhat.

I downloaded Lusted's AI for Kingdoms, and somehow combined it with the latest version of the DarthMod. I forgot the exact combination, but I ended up with about 7 files that I just copied into each of the "Data" folders of my 4 Kingdoms campaigns.

Now the AI isn't exactly brilliant when it attacks my castles, but it now always makes it to the very end (doesn't struggle to reach the 3rd wall of the Citadel anymore), makes aggressive use of combining ladders, rams and siege engines, and tends to storm multiple sides of the same wall at once. This is on Very Hard difficulty, of course.

It's kinda fun to defend now! So I call that an improvement.