Maybe im crazy, but have you tried using a mix of AI types when you play a game.
My single player games are usually on tower maps (usually width wise, with a bit of water) with 8 players. 4 AI, 2 AI+, 1 AI++, and myself.
It lends itself to a more challenging game. Though theres only so challenging the AI can be. No matter what level it is, I can build good divisions, sit them on the front line, and merely collect formation experiance till the point im unstoppable.
No matter what bonus or lack of bonus the AI has, nothing short god himself granting AI sentience, its never going to be "smart". Its not going to extrapolate data about what you MIGHT do, only what you have already done. In that regardm no matter the bonus, all AI is predictable, and easily beaten.
But do at least try a mix of the AI types in a single game, might find it fills the gap between AI and AI+.
If youre not using the resource and leader mod. Yer a girly man. Try that mod vs the AI and cry like a baby.