With that many GR covers in your queue, I’d honestly still champ him and add the covers. Boosted 4s do more match damage than level 255 5s, so on boosted GR weeks, you’ll at least get that benefit.
You have 2 2*s you could sell without impacting your DDQ chances (no reason 2Marvel, MNMags, and 2Daken can’t handle it all) in the event that you come across an extra Rulk cover or if you’d like to bring him to max level ahead of that anyway.
Since you don’t have any Rulk covers waiting, that would be my strategy. Max GR, gather what ISO you can to bring Rulk as high as possible for now, then scramble for ISO if/when another cover hits.
Btw, “tier” value is still somewhat subjective. Sure, Iceman, Rulk, Hulkbuster, Peggy, 4Clops, etc. will be stronger overall, but most characters with offensive value can carry their own weight. Fury was my 3rd 4* champ, and I thoroughly enjoy playing him - his boosted 5 cover yellow does 8k single-target, 3k AoE, stuns, creates crits (that hit HARD with boosted 4 damage), and creates seriously beefy protect tiles if you can fire it at full strength - and in nodes with high health opponents, you will get to do so quite easily.
Mr. F and IW in their current state aren’t worth it really, but most other characters have significant offensive punch in boosted weeks as long as they have damaging powers.
Once you have an upper-middle to top tier 4* champed, the overall strength of the next 4* champ starts to matter less. For reference, I have Thor, IMHB, Fury, Star-Lord, Ant-Man, and XFW champed, and they all see play time, even unboosted.