New here and late to this game and my longish post disappeared probably in the best interest of all.
1) OpenXR and the videos from Pie in the Sky Tours on youtube are critical to dialing in your settings. much easier and quick to change things there imo. Also easier to dial things in.
2) It's easy to pop in and out of VR but it does take a few seconds. It seems best, imo, to not need to leave to change things while flying so in that regard...
3) I have a button box for common and easy to remember changes like flap, ident, COMMS changes, etc. But I have migrated to using the mouse more often than not. It's in reach, holding the left mouse button down and moving the mouse makes changes to the dials, etc. It's easy and as long as your mouse in in reach, it's nearly as quick as having it mapped to a button box.
I have the HP Reverb Pro, it's 3 years old and simpy works fantastic though I'm sure more modern HMDs may improve detail. While I have a 12700k and a 3080Ti, getting frames dialed in took me some time. At O'Hare last night in the 172, i had about 45fps while taxing. A short hop around Chicago and my frames were high 50s.
The HMD is limited to 90fps and while racing online in iRacing getting 90fps was critical for smoothness but that seems to be less the case with msft flight sim. While I'm still focused on getting high fps, there is a level of detail that I was able to dial in using OpenXR seems to have reduced by "need for fps" to exclusion of all else though I'm certain there are things I can still do to improve it.