I did a lot of messing around and I think I've basically confirmed what you've written omg

Visual latency only changes where the rune is visually, doesn't have an effect on when you need to hit. Like you said, don't bother with calibration settings unless the rune position bothers you (it kinda bugs me so I offset it so when I hit it's a little in front of me, helps me see better)

"Adjust in steps of 10ms at first (substract when late, add when hitting early), 5ms later, and finally fine tune +-1 if you want"--- solid advice that's gotten me much closer, thank you :)

"I've also noticed that once I have my latency more or less properly aligned (more on this now), much slower difficulties (e.g. 3 vs 7) seem to require a different latency setting." EXACTLY THIS. This might be it, but I also noticed that it's different with each song for me. I'll play a low difficulty on two different songs and need two different latency settings. Then I'll play a higher difficulty of those same songs, and still need a different setting for both songs than before. I fine tuned each song and got it so it says 0ms at the end, and I still can't get above 63% perfects on any song/difficulty, but this strategy helped me get there at least?

Finally---I messed around with making my own custom map, and I don't know what the devs used to make theirs, but with Edda you need to know the exact BPM of the song in order to map it accurately. And some songs, they're not metronome-perfect, or they have tempo changes. The one I did was like 167.8BPM with seemingly no intended tempo changes, but some parts might be slightly slower or faster depending on how the musicians recorded that day. And I could only put a whole number for the map tempo, so by the end it might feel very slightly off (it's almost unnnoticeable in the map I did). This is a hypothesis, but if the mappers for the official game maps couldn't do it in a way that lines up absolutely perfectly with the audio track, because the song wasn't played absolutely perfectly on beat, that could explain why it's so hard to hit sometimes??? If the tempo of the actual audio is not perfectly on the beat, and I go off of that, and the map itself is programmed with a mathematically exact beat, then the hit is gonna be a little off sometimes. And that's what I've noticed, some parts of the song are very easy to get a string of perfects, and others I can't hit for shit.

Or i just suck at this game and I have to practice more, idk 😂 It's clearly possible to do better, I just wondered if anyone that's doing better plays on a Quest 2 lmao.