From my point of view it is simple ;-)
As I am using Pimax headsets, too, I think openXR will not work or at least using PimaxXR will better to use.
https://github.com/mbucchia/Pimax-OpenXR/releases
Just download the latest release and install it - the quick setup is also documented in the README.md of the git repo. I am using a 3080 and I have a gain of 3 to 5 FPS which is a lot when using VR headsets.
And yes, you can just switch between PimaxXR and SteamVR by just clicking the option (radio button) in PimaxXR - I think SteamVR needs to switched off while clicking or needs to be restarted.
edit: just to mention ... Pimax Client Software does make no difference to PiTool in my setup and I experienced a FPS loss while setting "fixed foveated rendering" to any value different to "1"