I don't think I'd like a boss rush. Especially as you describe it, I think it would be boring fighting the bosses over and over again with just new, inflated statistics each time.
What some other CRPGs have done, however, is have something like a gladiatorial pit, or some kind of very combat-focused campaign. The idea is that you start with a level 1 character and your teammates, and you go through a whole bunch of battles and level up that way. There can be some kind of story in the background, and even some new characters for you to interact with, but because the campaign is so combat-focused it's much easier to design than a full campaign. You basically have to concentrate on making interesting enemies, and interesting arenas for them to appear in. All equipment is whatever you can buy from some vendor *and should really not include massively OP legendary items, Larian, the idea of this would be a combat challenge.*
You could still have room for non-combat skill proficiencies by having characters that will help you out if you persuade them in between battles, etc.
