If you play (primarily) SP it’s understandable you wouldn’t care about balance too much. Only those things the AI was able to exploit would need to matter to you. And as you argued yourself, you’d be representing the majority of people who own AOE2.
At least I understood your protest against 2-pop units from this perspective, but I could have been wrong.
Without bloodlines, a Chevalier in Castle Age is like a slightly better FU Knight. Might remember wrong though. If that’s still too strong you can take away Scale Barding armour and/or Iron Forging. It’s definitely feasible in some way with some tech tree balancing I think.
Not needing houses and being able to build a second TC in Feudal was completely broken too. It’s pretty much fine now.
Either you end up giving them a way too strong option (+2/+2 cavalier in castle age would just be overwhelmingly good and that applies to 1v1 too) or you end giving them a really bad option by removing part of the tech tree.
Those are totally different things from 1 more upgrade in the castle age stable to one of the most common unit in the game. You are talking about +20 hp and +2 attack. You are giving them one of the most strong power spike in castle age (and no, capped ram for cuman is not even close to this).
Stick to SP and leave MP alone man
Did you ever wonder why all the Battle Royale Games from PUBG to Apex Legends don’t have vs A.I.? It just doesn’t feel the same way since that game mode is purely designed for a Player versus Player or a multiplayer environment.
Thus I wouldn’t really consider playing a Battle Royale vs A.I. mode as “Singleplayer”.