You’re a 2-star player with a lot of them champed. That’s good. Keep them all and continue applying champ levels. Get your Ares and Cap champed when you can so they’re rising in champ levels also. The champ level rewards from your 2-stars are going to help you advance to the 3-star tier and also provide you with some ever-coveted HP so you can expand your roster slots.
When your 2-stars reach max-champ status (Level 144) keep the ones you rely on to win matches and sell the ones you don’t. Restart a new version of the ones you sell and work on getting all those champ rewards again. Once you’re firmly established in the 3-star tier you can do this with all of your 2-stars as you won’t be relying on them anymore. Repeat this process into infinity.
While you’re working on this, keep building up the covers on your 3-stars. Since you have limited roster slots you may need to make some tough decisions on who to roster and who to let go. Prioritize rostering, covering, and champing the top-tier 3-stars and let the lower-tier 3-stars go until you have the slots to roster them at a later time.
Top-tier 3-stars: Captain America, Cyclops, Doc Strange, Iron Fist, Iron Man, Luke Cage, Kamala Khan, Magneto, Scarlet Witch, Thanos, Wolverine, Hawkeye, Deadpool, Thor.
Low-tier 3-stars: Daredevil, Gambit, Gamora, Ragnarok, Punisher, Captain Marvel, Mystique, Vision, Doc Ock, Sentry, Colossus
Everyone else is somewhere in the middle.
Avoid acquiring 4-star and 5-star covers that you can’t afford to roster right now. Do this by hoarding your CP and Legendary Tokens. Let it pile up because they can’t help you until they have 10-13 covers anyway. Rostering and champing ALL of the 3-stars should be your current goal. Once you’ve done that you can start using new roster slots to start building fours and fives the same way you’ll build your threes while in the 2-star tier (and you’ll find all those LTs and CPs you hoarded very useful). In the meantime, don’t put any ISO into your 4-stars (or 5-stars!), wait until you’re ready to make the transition to that tier by champing them when they have 13 covers.