Running endeavourOS with a 3080 and Plasma since 5.25 (on 6.1.2 now). Never ran x11 and I don't intend to ever again. My experience is mixed, so to say. There are a few things keeping me on Windows but (very) slowly I'm getting there.
About HDR. I didn't need to do anything extra to get it enabled on my desktop, simply toggling it on settings works.=, and I can also get mpv to work with some tweaks so I can watch films. On games I would need gamescope to run it, but that comes with a set of issues like not having Steam Overlay and Input, so for HDR games I run them on gamescope without overlay, and every other game I just run them normally. I also have a lengthy writeup on trying to get gamescope to run with the Steam overlay, but ultimately it's one or the other right now.
On NVIDIA drivers, for me the 555 and explicit sync patches have made things worse. I never had issues with things flickering on 550 except for Electron apps, which would flicker and have awful input lag, but that's easily fixed by setting ELECTRON_OZONE_PLATFORM_HINT=wayland on my /etc/environment. The issue with 555 drivers is that there are some VRAM leaks happening. They fill ridiculously fast, even just dragging a window will make kwin use 2GB of VRAM. Since there is no shared VRAM at all on NVIDIA Linux, as soon as I hit my 10GB cap, Xwayland will crash along my game and Steam, and sometimes my desktop too. On 550, I would only get framedrops for a while.
I should also note that the proprietary and open drivers have no difference at all for me, and enabling or disabling GSP firmware also has no difference.
Lastly, VRR. It will not work at all if you have more than one monitor connected and enabled on your NVIDIA card. A workaround if you have a second GPU (or your CPU's iGPU) is to plug your extra monitors there, and then VRR will work on your main screen. A second option would be to disable your extra monitors anytime you would play a game, but that's not ideal at all.