I have heard symphonium is very good if they are looking at closed source Plex anyway. It works with jellyfin and navidrome.
I just use syncthing to sync all of my music to my phone's SD card. Then PowerAmp since there aren't many fully featured foss music players. I am keeping my eye on Auxio though, keeping it installed and updated.
That is a different usecase though. That is simply syncing local musical with a server.
I do that too because i have an SD card. Just use Syncthing for that. Much faster and less hassle. You can use any music player on your phone that you want, not just one that works with jellyfin.
If you aren't streaming music in real time for the majority of time, then do a phone sync, not a streaming server.