Just mount the ‘remote’ file system via NFS (or samba, if you must) and play the media on the ‘local’ machine as if it were a local file. Done. What am I missing?

I use mythtv (FOSS) and it’s fine as a PVR (schedule, record and playback broadcast TV) with a SiliconDust HDHomeRun network-attached tuner. It also has playback for downloaded files but I prefer to use mpv for that. Mine is still connected to the internet and I use firefox/transmission for youtube and other functions.

No idea about plex or jellyfin.