A few questions

For your first question, I wouldn’t necessarily prioritize selling unless it’s 89-90’s that you are already interested in upgrading.

but I would prioritize building up your coins because there are so many cards and promos dropping in December. all the cards you’re eyeing now will 9 times out of 10 not be the best card at that position for you anymore and will cost 1/2 of what they cost now if they do remain meta (not referring to crystals and team diamonds) definitely have coins to spend throughout December that’s probably number one priority.

For your theme team question, even if you’re not rocking one nowadays you should still be rocking one. All of the S3 free players, basically all of the S2 free players, the two shapeshifters, the crystals and their reduxes, the max upgraded breakouts free players porter and Thornton, and the right tackle John madden coming tomorrow all have 32 chems. I just named 15 players you can set to one chem and you can throw 5 70ovrs as your o-line subs for another 5 to get +1 speed.

Now to actually answer your question, tt+crystal is the name of the game now so yes, a theme team is the best way to approach it. However, if you don’t like committing to one team’s roster of players, you can get away with having partial theme team that you can adapt to whatever team you want based on the players you have in that given time. Example: you have a Seahawks theme team and they drop Barry sanders you really want, so you go through and switch all your 32 chem players to lions chem and then buy core lions players for your subs to get Barry and give him theme team boost.
Final Piece of advice I’ll give is to always put 70ovr players at o-line sub first, because your defensive and skill position subs will end up being your kickoff and return team. The favorite team thing is totally up to you, it’s really just how you want to play the game. Any theme team is an advantage over a non theme team, especially when they finish filling out all the positions for each theme team