As I see it, the problem can't be fixed, by giving the AI buffs. The AI has to engage with the systems of the game more effectively, i. e. either the AI starts stacking maa/knight-modifiers like the player does, or maa/knight-modifier-stacking is heavily nerved.
I tend towards the latter one simply because it's probably easier to implement from a developer point of view. Judging by how the AI acts in Paradox games, it's probably insanely hard to get it to do any useful thing.
Perhaps part of the solution could be to make army composition more important. Right now it feels like it doesn't matter if you have little horses or little bowmen, as long as you stack the modifiers, you will win most battles. Maybe the counter efficiency could be tuned up a lot, while the bonuses are tuned down.
You could also borrow a little from Imperator Rome, where the difference between levies and Legions is mainly that you can't choose the composition of your levies, rather the game just gives you a composition based on the culture. This could be implemented into ck3, so most of your levies would always be footmen, with there being some cultural traditions, which offer bonuses like "+10% Levy Horse Archer Share".