Balance in this game is challenging to measure. If there were a public DPS meter, you and your party members could gather real-time metrics to help you (and the devs) hone competent builds (and develop what's vital to performance). If there were better/more reliable IT infrastructure, then quicker hands and incremental build choices might provide repeatable and dependable outcomes. I don't see these being addressed in the short term, so we're all (and the devs) left with balancing the game given the tools available now.
First, as the OP pointed out, I see so many different builds leveling without problem or complaint that I don't think this is anything but an endgame discussion.
If you give maxed-out, endgame casters the same DPS power they had before the ham-handed nerfs, I don't think there'd be any good reason to bring most builds into your endgame reaper groups anymore. Two casters who have some healing capability and perhaps a token melee for bosses/agro could crush R10s just fine. If you were in a full group with a great DPS caster or two in their heyday, your new job as a triple completionist maxed out melee with goated boosted gear would be breaking boxes and pulling levers. That's what got us here.
Without giving dps casters so much back that there's no reason to play anything else, I think a measured approach would be to remove the R7 - R10 nerf and make sure every dps caster has a delete mob button (if built properly, of course). Requiring five casts to delete an R10 mob when an adren barb can cut it and two more of their friends in half with one swing will forever perpetuate balance issues.
No, the answer is not to nerf the living spit out of anything remotely fun. Let the barb cut the thing in half, but then, while they're swapping weapons to trans, proc prowess, and rerage, let the caster blow up everything in the next room. It's going to take subtle and nuanced changes to get the balance to feel right (feel the operative word, as there is no proper meter to prove otherwise). My earlier
proposed list of detail-oriented players with doctorate-level DDO knowledge contains the right people to help massage those relatively innocuous adjustments into meaningful and noticeable balance change.
This isn't a definitive list; more than half don't have the time for it anyway. But these are the types of players that we need to help the devs build for future growth.