I think it’s fair that levels that reward mythic track crest/gear like 9-10s feel comparable in term of difficulty to what mythic raids are which I think they currently do feel like. I like that like mythic raiding the difficulty does encourage people to build reliable groups to clear them.
For me the +10 tuning has been something I really like about current season with combining fort/tyr which makes dungeon more interesting as when it wasn’t tyranical bosses just fell over and when it wasn’t fort trash just fell over and it means I can use the same route every week.
The lower levels tho as they are competing with delves don’t really feel rewarding as much so I feel they could be tuned down.
I like the concept of challenger’s peril but like you said it is a bit more punishing toward pugs. Which to some point a tradeoff could be to switch to instead of losing time the key simply depletes after 10-15 deaths which would still force you to play the dungeon well but wouldn’t make the few deaths here and there as punishing.
I do like that challenger’s peril make it so you can’t really time keys with 20+ deaths which was something that happened a decent bit in DF, it didn’t feel like I earned “timing” a key when that happened but it still happened.