I totally agree with your thoughts on the AI. Currently, it seems that the "Very Hard" setting isn't as challenging as it should be for pro players, and adding more strategic depth would definitely make it more interesting to practice against. The points you've listed, like mismanaging units and poor strategic decisions, are pretty noticeable, and fixing these would make the AI feel much more like an intelligent opponent.
If the AI could adapt more dynamically and adjust its strategies based on the player's actions (like making larger pushes, better use of formations, or knowing when to retreat), it would add a lot more depth to the game. The idea of giving it a cavalry or archer rush feature sounds like a good way to diversify its tactics and make it a real challenge.
Increasing the gather bonus to 100% could be a good start, but as you said, it's not just about the resources—it's about how the AI behaves in combat and strategy. And I like the idea of the "Very Easy" AI doing small rushes; it would definitely help beginners get better without feeling overwhelmed.
Hopefully, the devs take note of this, as it would really improve the game for higher-level players.