The bottleneck is the 90+ players and not really a bottleneck. I built Metcalf today, Day 2 without using any MC or coins on cards. Stamina and Gems only.
Not really the math but it should still help: https://www.muthead.com/forums/madden-nfl-mobile-discussion/3606-psa-combine-promo-strategy/
Probably the stamina. As far as the players, if you wind up with excess badges, they can be used to trade in for players
F2P... Not sure what the bottleneck is yet and haven't done the math. I haven't dumped any gems into the extra Combine stamina deal. I've just been playing first win and letting the stamina regen...
I've been doing mostly TS (spending a few hundred gems a day to redo the event) and the grind for one TS master (Derwin) seems fairly easy (points rewards, division sets, achievements and a couple season games a day) but I'm not sure what to do with the TS Playersright now and I'm not sure how possible it will be do get both a TS Master and a Combine grand master.