I don’t really play for maximum competitive points, just full node rewards (and subsequently event progression) in PVE and full progression (1200 points ideally) in PVP, at which point I’ll generally save out so I can hang on to season points for season progression, which is pretty often good for t50-t100 per event. I’ve found that playing this way the required character is almost always a non-factor unless they are very good, and even a max champ bad character is a non factor as in the case of my max-champ Frost in her PVP in the face of boosted 5* characters.
I keep a single 1*, juggernaut, on roster as built as he can be as a 2 color character. he can solo the DDQ node, though I find he is more board dependent than 1* spider-man or iron man who can also do it, you’ll just never use his blue. I throw away several yelenas a day from PVP rewards, and I’ve considered rostering her just to have all the limiteds, but it’s a pretty dead roster slot since 1* can’t be champions.
I keep all 2s on roster, even a champed bagman (i’m on my 4th iteration of him so far i think?), and I only ever have 3 of them (non bagman) champed at a time. When one of them hits 144 and I get a replacement cover, i flip it and reroster the one cover and then champ whoever has the most saved covers. I find that any 3 characters can win the DDQ nodes, and I used to do it with a solo 2 Thor. I have teams I prefer, but not so much I need to keep them maxed out. Playing this way makes my farm resource aquisition very lumpy, but it means that I have iso-8 stacking up to champ higher tier characters rather than always getting siphoned off to keep those more rapidly flipping ones champed.
I keep all 3s on roster, and flip max-champs as soon as I have a single replacement cover. Using the loaner is just fine if you have 5s that are flanking them. This tier for me only exists to unlock nodes in PVP and give rewards. I only sink iso into them when they can go directly to champ.
I have all 4s on roster, all are champed except the 2 most recent and a recently added Frost dupe. This is so far the only tier I’ve rostered duplicates on, because it takes so long to max-champ a 4. I do think that there is at least some marginal PVP value to having a high leveled 4* required, but it’s become a lot less now with 5* boosting being a factor for me. Similarly, should they ever decide to apply retroactive feeders into the reward tree, having max-champs here has historically been the only way to take advantage of that, but I do think it is highly unlikely to happen again. Though I could see a sweeping reward change should a hypothetical 6*, or some change to Supports or something ever goes live.
5*s:I just have one of each, but that’s because I didn’t start seriously aquiring them until Saved Covers as a feature was already a thing. That came online RIGHT when I landed my first 6th cover of Daredevil, so I lucked out not having to roster a dupe way back then.
At all tiers I only spend iso-8 on fully champable characters all at once, I rarely to never invest in partial builds now that SHield Training doesn’t require it. This gives me the flexibility to pick and choose who I champ based on either needing them mechanically, or cashing in on any saved covers they may have banked up. I wouldn’t say I"m iso-positive, i’m probably iso-neutral in that I can champ anyone I need to, but it usually drains me down to 0 in the case of a 5*.