The problem is that PlexAmp is literally the killer feature of Plex. Literally no open source software comes close. It would be great if it did, and I would switch, but it’s the only app that even remotely competes with Spotify for me for that reason.
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.
Finamp isn’t there yet but it is closing the gap to Plexamp.
Never used PlexAmp but I'm happy with FinAmp