Some further testing yes I think it works. It's just that Plex has a bug i think. On emby it fully works.   Played 3 4k HDR to SDR transcodes and a background Plex optimize encode at the same time. No crash or anything.   Why I think Plex is the problem now is that Plex is transcoding at 10-20 fps and needs burned in subtitles to have no artifacts. Emby is working perfectly while transcoding HDR to SDR at 160fps.   Running on an 12400 btw.