HDR is just a marketing term for software that increases the colour and black saturation of content beyond the actual input. For photography that means more saturation than what you see in real life, for video that means tweaking it after you receive the signal.
An OS can't "support" or not support HDR; if a video has HDR and your display has the dynamic range then you'll see it in "HDR". Unless you actually want your OS to post-process everything on the screen to have marginally higher contrast than normal?
https://support.microsoft.com/en-us/windows/what-is-hdr-in-w... https://www.pcworld.com/article/546269/auto-hdr-deep-dive-ho...
> Setting up multiple displays still requires fiddling with xrandr, xinerama etc.
Dunno what you're doing but you can literally just plug in another display and it'll work... And per-monitor scaling works on Ubuntu/Gnome.