I’m login for an app that is like iTunes that does music syncing for Android devices.
- Main priority is syncing them based on playlists, along with the playlist themselves
- a friendly GUI
- (optional) wireless syncing
If you have possession of the files, I would try plexamp. Feature packed. Allows me to access all my music on my mobile devices, as easily as I play music on my desktop.
It may not be what you want, but It’s worth a look:
In this same way I always recommend Finamp with a Jellyfin server.