As a new coach, now that recruiting is over, which of these areas should I be placing my focus on? What's the best way to figure these out, trail and error after each game?

I would say for formations, do something you personally like or something that sounds fun...playbooks and gameplan is definitely something you will have to trial and error or have a mentor who can kind of guide you.

and after you get all that down, then i would then worry about the other settings like aggressiveness, blitzing, etc. and, yes, alot of that is just trial and error.

it's also just a lot of fine tuning and you can compete without knowing all of it. or even most of it. the main thing is player talent... recruit it, grow it and keep it maxed in game and you'll be ok...