I’d try Jellyfin but I don’t know if it has a good equivalent to Plexamp. I saw something called “Jellyamp” but it looked like a desktop-only app, and seems abandoned now.

I’d be looking for some of the fancier features that Plex and Plexamp have, like their sonic analysis (machine learning analysis of your music) that can suggest similar songs, fades between tracks, gapless playback for albums, etc.