Valve has truly headless runner (gamescope) for Linux, e.g. you can start it manually from command-line over ssh

  gamescope \
    --backend headless \
    -W 1920 -H 1080 \
    -w 3840 -h 2160 \
    -r 60 \
    --cursor-scale-height 1080 \
    --force-grab-cursor \
    -- steam -gamepadui -pipewire-dmabuf

But it has many issues compared to Games on Whales, for example, which is much more efficient and scales cursor better.

https://github.com/ValveSoftware/gamescope