Thanks, appreciate the write up. Definitely sounds like HDR under Linux has a long way to go to reach the “just works” level.
It’s not as long as you might think, by the end of the year we should have out of the box HDR on Linux. At least for Proton games. All the puzzle pieces are there, they just need to be put in place.
could you use gamescope and mpv under Gnome and get HDR support or is KDE’s HDR support essential here?
No, not until GNOME implements their HDR support. You can however run gamescope and mpv directly in tty instead of KDE/GNOME.