Thanks for the response, excited to see more support being added!
You can run Steam in gamescope, doesn’t matter if it’s sandboxed or not.
So, installing your Vulkan layers and gamescope-git on the host and running ENABLE_HDR_WSI=1 gamescope --hdr-enabled --hdr-debug-force-output --steam – env ENABLE_GAMESCOPE_WSI=1 DXVK_HDR=1 DISABLE_HDR_WSI=1 flatpak run com.valvesoftware.Steam -bigpicture should do it on Plasma 6?