Recommended OS for Jellyfin client

[Host-in-the-Shell]

Well, provided that you tried different Kodi interfaces and liked none of them (and I can't blame you, I moved away from it for the same reason), and HDR passthrough doesn't matter, then I'd try something like a minimal Debian install, setup autologin without a user password, autostart some minimal display session like sway and configure it to start Jellyfin Media Player fullscreen, which you can then pair with a controller.

I don't have a pi or any arm device, so I'm not sure if sway (or any other software) works under it, but I suppose it should.