Yah, I know it's old content, but I just ran it again for the first time in a LONG time on EH. I had never tried soloing these on any epic difficulty above Epic Normal.
The one thing I've hated most about this quest is the timer mechanic - waiting around for the next wave. On heroic levels except Elite (assuming you're at level), you wind up just letting potions and spells tick down waiting on the next wave.
I just ran this on EH (first) then EN... IMHO EH is a nearly perfect balance of difficulty! I'd go so far as to say with a slight uptick, EH should be this difficult in quests across the board.
In EH DA, timing is right, the mob difficulty is right, the rewards are probably ~.25 tokens high, but that's not OP, so I can't complain.
The last two red named leaders of shavrath are bags of HP, so they could use some tactical AI upgrade and slight HP nerfage. Once you stop the one from teleporting uncontrollably, it's only a matter of staying out of his claw range - decent reflex saves keep you alive easily.
But the second and subsequent waves are nicely difficult to a character 1 level below quest difficulty - wave 4 and 5 almost took my squishy arty out several times (thank god for scroll healing).
Most EH content relies solely on HP bags to "increase difficulty" - that's not increasing difficullty, it's increasing caffeine intake. EH DA has a selection of mobs that causes you to have to think more tactically - and that's what it should be - training ground for EE. have to keep moving to avoid frakkin' bezekira trips, but you'd rather sit still and plink the warlocks. Have to keep ALL resists up, but also have to remember to block the barbazu glaives (or whatever that is they're swinging). Very nice!
After finishing that one, I ran the Deneith chain on EH - yawn - back to overinflated HP as the sole concern. Only one mildly interesting was Tide Turns, and that was because the map design and traps, etc were the feature.
tl;dr: I sincerely like how you've tuned EH DA, tune it up just a touch and use it as a model for other EH tuning.