the physical display output is claimed by something else totally separate.
Anyway I did figure out how to the headless backend working, but I did not figure out the udev rules for this headless session
Allows Sunshine to access /dev/uinput
KERNEL=="uinput", SUBSYSTEM=="misc", OPTIONS+="static_node=uinput", GROUP="input", MODE="0660", TAG+="uaccess"
Allows Sunshine to access /dev/uhid
KERNEL=="uhid", GROUP="input", MODE="0660", TAG+="uaccess"
Joypads
KERNEL=="hidraw*", ATTRS{name}=="Sunshine PS5 (virtual) pad*", GROUP="input", MODE="0660", TAG+="uaccess" SUBSYSTEMS=="input", ATTRS{name}=="Sunshine X-Box One (virtual) pad*", GROUP="input", MODE="0660", TAG+="uaccess" SUBSYSTEMS=="input", ATTRS{name}=="Sunshine gamepad (virtual) motion sensors*", GROUP="input", MODE="0660", TAG+="uaccess" SUBSYSTEMS=="input", ATTRS{name}=="Sunshine Nintendo (virtual) pad*", GROUP="input", MODE="0660", TAG+="uaccess" SUBSYSTEMS=="input", ATTRS{name}=="Sunshine PS5 (virtual) pad*", GROUP="input", MODE="0660", TAG+="uaccess"
this is....not enough