Thank you very much for your tries. For my current projects I will stick with the old engine version. In the long run an internal stereo viewer, simply based on device rotation without any vr/xr/polyfill should do the job, even with better customization:
Recently a pc user shared an initial solution with me, unfortunately he didn’t allow me to make it public available. But as soon as I find some time to get into shader stuff myself I will probably work on such a solution from scratch.