So on one hand you want it to be higher level on the other hand you want lower levels to be less interresting?
Id like to point out that Descent into Avernus lts you go to literal hell at a pretty low level.
IDK about the current 5e setting, but in older Underdark Books, the thing was pretty much seperated into multiple layers. Which were different in level range, so unless you waltz into a Drow city you probably got a fighting chance
