I think some of the problems run deeper than balance alone.
The game could be very hard and still fun, except for the extreme grind and gating of the game behind progression. That ultimately makes losing ULTRA SUCK and feel awful, when losing should be almost as fun as winning. If losing didn’t suck so much, you could nerf things a lot more without mudering your players’ happiness.
I wrote some thoughts on loss aversion here:
The other thing is that you need more degrees of freedom for difficulty than just damage and quantity. Density discrepancies make things useful in legend / cata+ but useless anywhere else. Ginormous health pools make instant kills more powerful at high difficulty than at low. Thp on kill scales badly as enemies become beefier.
One degree of freedom might be enemy attack speed, or if that’s hard to increase due to network delays, slower ally animations. Maybe cataclysm has the same health enemies at legend but you’re running at -10% attack / block speed. Maybe player outlines are disabled. This is just random stuff I can think of right now but there are ways to approach balance that don’t necessarily creat the weird breakpoint centric density dependent metagame that we have now.
Edit: another idea: change aggro ranges per difficulty.
Edit edit: you can also bring the base difficulty way up and do things for recruit - champ that make it easier beyond health and density values. A visual cue to block maybe? To dodge?