Help please - new player

New to the game, started playing about a week ago. Current roster attached. I didn’t think I'd enjoy it that much so wasnt thinking strategy wise to save coins, gems, etc. But now I'm hooked and am looking for a crash course in strategy. I've manually played about 25 games so far.

Main questions: Positions to upgrade? How to best use XP upgrades? Skills to upgrade? Focus on increasing bronze players or getting everyone to be at least bronze to then get upgrades?

Much more later.

TIA!