I mean you have a really solid collection built up, my main advice is to get some easy going buddies and just start playing. That way you can start to relearn, and it's not at all competitive and you can use house rules to make your way around warband rules and such.
Of course eventually you wanna get them 'legal' but learning the ropes using older rule sets or modifying warbands to fit your needs. Then as you get acquainted with the game, tactics, formations, etc., you can start forming up armies.
Sorry I didn't really help with your main question, as I am in a very similar boat with lots of old models, rekindled interest, and a loss of army formation knowledge. But I found starting small and easy with a slowly growing gaming group of close friends has really improved both my skills as a tactician and army builder