Okay, it's the weekends so I finally have the time to do this at my leisure.

Variant Dungeon
It's fantastic. Very enjoyable to do solo or duo with a friend for the purpose of exploring and puzzle solving.
While I appreciate having different routes depending on your choices, and needing to collect all the notes from them, I find that a couple of notes are not as meaningful as the others. One example for this is Route 8 and Route 9 (one drops boulder, one makes wind).

I know it changes the boss mechanic, but I do wish it doesn't count as a separate note. There is no immediate difference for them as we still arrive at the same location regardless. Route 1 and Route 2 does this right. It leads you to the same path but, one requires you to sidetrack for a bit to discover an extra door.
That's my only very minor gripe. Other than that, I had a great time.

Criterion Dungeon
I went in with my FC. None of us do Savage so we were on average i610. The trash mob caught us by surprise -- they have more HP, they hit harder, plus they do their signature aoe which we generally can recall from their overworld counterpart. I like it tbh. Nothing too hard. We just can't happily spam AoE like in normal dungeon.

The first boss was rough especially since we went in blind. We tried for about 6 pulls. Managed to learn up to the ice stack before deciding this is way too hard and not worth our time. At least not for doing it blind.

I personally would like to give it another shot with a proper group (and maybe with better ilvl in the months to come). But I do agree that the mechanics and the pacing feels like a savage fight. The boss fight is a HUGE jump from Variant to Criterion. Trash mob feels just right -- well, the 1st set that is. I don't know how it is past Silkie.