Has been a while since I used a Pimax headset TBH, I only have an older model here. Pimax has always been a bit iffy to handle due to their intermediate software though, so what you describe doesn’t really come as a surprise.
I’d suggest to try SteamVR first, that’s how I tested Pimax back then. vorpX sets specific headset sync settings for Pimax in SteamVR mode that I figured worked best (for me) at the time. Not sure whether these settings are still optimal, but that’s definitely worth a shot.
I never tried PimaxXR, so I can’t really comment on that. The key for getting Pimax headsets work as good as possible has always been finding working headset sync options in the vorpX menu. That’s probably also true for the PimaxXR implementation. Might generally make sense to use a FPS limited sync method that leaves some GPU headroom for whatever the Pimax app does after vorpX.
On a sidenote:
The 24.1.0 cutting edge build linked on the front page has revised sync handling with more logical sounding presets and more FPS limit options. Trying that is heavily recommended.