

Dumb idea: if the configuration is on a file, you can make it immutable. Something with chattr +i can’t remember. I take no responsibility if your server catches fire, and you may have to manually set the flag on and off if you want to add new media/change settings. It may be more complicated if everything is inside a “full” sql db
As for the rest, either start jellyfin at boot or a custom launcher or kodi. I don’t know if plasmashell is still working, it can probably be configured to only show the correct stuff. There are also custom launchers for android tv

Lenovo Legion. If you pick a configurable one and remove windows they’ll lower the price accordingly (sometimes 100€ for windows pro). Some models offer Linux pre-installed.