Any SBC like Radxa Rock (cheap) to Orange Pi (powerful). 4k output, a linux you control, play retro games or stream torrent movies.

Not well.

If you only care about file playback, bit still care about things like HDR and quality audio I would recommend something like the OSMC Vero V instead.

It's still relatively cheap, and open source, but also let's you playback something like a blu-ray rip without loss of quality.

It has the other bonus of not requiring any real setup or maintenance.