Learning the game in earnest

I hail from what is probably the archetypal AoE2 background. I played the game a lot during my childhood but never really learnt it. I always opted for 8 player free-for-alls against Standard AI, turtled, built a huge army, and steamrolled everybody else with Champions and Paladins. I'm not entirely sure but I think I never lost a game... A few times I tried against the Moderate AI, though, got rushed, and rage-quitted.

Anyways, I've finally started to practice the game seriously. I enjoy the new civilizations and watching professional competetions from YouTube. My objective is to be good enough to beat the Hardest AI consistently; I'll probably switch to multiplayer at that point.

Right now I mostly play Arabia with Byzantines, the latter because I find them the most generic civ with no significant eco bonuses and an open tech tree. I guess if you know your build orders with Byzantines you can utilize them with almost any civilization. (My favorite are the Malians, though.) I go for a simple fast castle build (sometimes with militia or scout rush) and then into knight spam.

I can quite comfortably rush the AI hard enough to secure a firm eco lead. My problem is twofold. First, I've heard that the AI 'is not supposed to be rushed', i.e. it doesn't handle rushes very well and should be allowed to boom. I do have, however, trouble with booming faster than the AI does. This is of course just a matter of practice.

More importantly, I can usually drop a decent number of scouts and/or knights upon the AI and secure the lead. However, I have problems with the transition from a stable or two to the numbers required for the late castle/early imperial spam. At that point the AI can usually recuperate from the damages and the game morphs into a prolonged grind before I have established a sufficient number of Stables or whatever. I still usually win because of the early lead but the games become kinda stretcted.

TLDR. I know it all comes down to practice (lot's of it), but I wonder if you have some general advice about the transition from small-scale early game to the larger scope of the mid-to-late game.