a 22 y.o MVP at SF with around 50 TSP and an 18 y.o perennial all star at PF and a couple other decent prospects at PF and C
I wouldn't bother to train the 22yo MVP, since he's already far behind and training will be slower because of his age. As already stated focus on 18 or 19 year old players. I'd say 18 yo preferably unless you can find a 19 yo with nice potential and skills.
Should I train only PF’s or should I do SF/PF(90%)
My advice would be to always train 1pos where it's possible. It's a lot quicker. Just make sure that you're giving 3 players 48 minutes each week and you'll be good on your way. Consider investing in the draft, since you'll likely be able to draft a decent player with all those bots in your league.
Other than that, just make sure to keep expanding your arena. It is your main source of revenue, and the larger it gets the more money you'll be making each week. Also make sure to set your ticket prices properly. Buzzermanager.com has a good tool to help with that. If you haven't already make sure to check out the guide and FAQ. Almost everything you need to know is written there.