By all accounts I think I have a fairly stacked roster, but I continue to struggle with more difficult content due to my own lack of consistent fighting skill (and lack of willingness to burn items/units/$$). As a result I've been slow to progress through post-Uncollected content despite frequent attempts.
I finished Uncollected a long time ago, took a year off, and have been back for a few months now. I've fully explored every Uncollected event since I've returned but it's always been a bit of a struggle and the final bosses have typically required a few revives each run. I've been slowly working on the completion of Act 5 - currently on 5.3.5 and keep screwing up the fights, restarting, and eventually giving up

. Same with Variants - I've tried the Polar Opposites one a few times, because Sentinel, but keep getting pwned by Thing, Korg, or Wasp on the first map - again, not prepared to burn items on the early levels.
So, 2 questions:
1) What bit of content should I be focusing on completing next?
- Act 5 Completion -> Exploration -> Cavalier
- Variants (Which one first? Polar Opposites or another?)
- Neither - keep doing Events & Arena in order to improve my roster
2) What (if any) rank ups should I be looking at in order to significantly improve my roster enough to help my get through the above content? I think I've ranked up all the valuable 5* that I have so far, barring some champs that really need to be woke (AA-ron, Void, Dr Voodoo) and all significant rank ups have capped at R4 as I'm VERY hesitant to R5 anyone unless I'm very sure it'll make a significant difference because I'm not doing high-tier Alliance Quest/War and so T5B are going to be VERY hard to come by. Alternatively I could be ranking up (and awakening - I have a 4* Generic AG) some 4* champs if they're going to be of significant use - options include Cap IW, Omega Red, Killmonger, Domino).