The GM end of things looks complex as hell. Any pointers on where to get started/tips/guides on how to be effective and not just rely on auto-pilot?
Basically, imagine WNFF sitting on your shoulder criticizing your every decision.
I had a steep learning curve since I jumped into the game after a 3 year hiatus, right into the multiplayer league vs. whytev and GuyfromCO. And my team's roster was total garbage. The first thing probably is to just go through as many screens as you can and learn as much as you can. Spend an hour or two if you're serious, just looking around at stuff and seeing what you can do. Check out the transaction/waiver page, the minor league page. Find how to set all your minor league lineups and rotations - I had a prospect almost derail because the game was automatically playing him at CF instead of 3B, and I didn't notice for a whole season. Learn how to sign minor league free agents and stuff like that.
The reports screen is your friend. There are lots of great reports - development showing you how your prospects are improving (or not), minor league system report showing you if AI thinks a guy is ready to move up, your financials & salaries in upcoming years, all sorts of stuff.
In my first week, I had two AAA OFs with the same last name (Paul & Tom Lund), one a top prospect and one a scrub. I accidentally waived the wrong one.
You can use shortlist features to designate, for instance, other teams' prospects you are interested in, if you are selling, or upcoming free agents you want to make offers on.
With leagues of fictional players, it will take some time to see how the game ratings translate into game stats, and what the league environment is. Pay more attention to stats than ratings, I would say, probably. Don't forget to look at players' intelligence and work ethic, and if they are a free agent, how greedy they are. My team sucks but we have a bona fide ace because he has "very low desire for winning".
In multiplayer, it also takes some time to figure out what trading prices & frequencies are, and what the going rate for FAs is, etc.