You still need X11 or Wayland somehow. I would suggest something that can run in “kiosk” mode. That’s essentially what you want - boot a single app and run that.
If you don’t care about wayland, just install xorg-xinit (since you said Arch) and put this in ~/.xinitrc: exec steam -bigpicture
Run startx to launch it on its own. There may be other dependencies you need for steam or for games, but you don’t need a whole DE.
If you want, you can even enable auto-login and then set up auto-start by adding this to your ~/.bashrc (or whatever shell you use):
if [[ -z $DISPLAY ]]; then
startx
fi






I feel like CachyOS should be further to Niche than Arch