P.S. You can't talk about great under-used dungeons without mentioning Kedge Keep. The underwater part scares everyone away, but the ZEM and some fun gear (eg. a super rare
chest that's essentially a 10 weight Bag of the Tinkers) rewards those who do brave it.
Also, if you haven't seen it yet,
https://wiki.project1999.com/Per-Level_Hunting_Guide has
tons of off-the-beaten-path soloing spots. They're not all dungeons, but still you might find some great places to try out there.