FWIW, for music in my car and on my phone, I only used Plex and use Jellyfin as a failover. I just use Pi Music Player and I keep my whole MP3 collection on a memory chip, so I don't have to be online at all. Whenever I pull the chip out of my phone and put it into my laptop to copy and remove my photos (cloud backup? no thanks) I update the mp3 folder.