Yeah I’m not a huge fan of the Nightmare Dungeons myself; the only reason to go through them is get glyph upgrades. It seems they rotate which dungeons become Nightmare ones each season, so I’ve been slowly working on a list of which I can go through the fastest so I can focus on other stuff.
iirc the Tree of Whispers is a guaranteed drop for sigils, but from what I’ve found they’re usually lower tiered then what I want so I end up salvaging them anyway.