Open source alternative to Steam inhome streaming / Nvidia GameStream

You can run Jellyfin on the Ryzen 3600 machine and point it to the NAS movie directory. At least to try it out (Alternatively, throw some movie files on the machine locally as a test). Then just use the Jellyfin frontend (http://localhost:8096/) on the same machine as the media player (or the official Jellyfin Media Player Linux app). Then also try if you like using it via Jellyfin app on the phone.