Of note: there is a difference in puzzle solving between knowing what the end goal is and knowing exactly how to reach the goal.
I think this puzzle was okay at the first (all the full moons in the light? in the shadow? If you didn't read the hint you might think that each circle should have a full lunar cycle)
and good at the second. It took maybe a couple minutes for me to solve once I started clicking, which is appropriate for a puzzle that gates off required areas.
Hopefully there will be more complex puzzles for optional rewards
There's multiple ways to reach the Underdark too.
I do think that there's a difficult line to walk tho when it comes to puzzles leading into required areas.
But I mean this is the feedback part of the forum, and my feedback is that I felt like it was too easy.
But I agree with the OP about the overall design of it outside of that and I thought it was great in regards to that too.
Peoples experiences are obviously going to vary tho but to me I just felt like it kinda wasn't a puzzle.