My combo is Jellyfin+Symfonium
With Symfonium you can manually download playlists and favorites for offline, and/or have a “rolling cache” where the most frequent listens are automatically kept synced for offline listening.
My collection is far too large to keep on my phone in its entirety, but with Symfonium I don’t need to, and if I’m ever caught without internet, I’ve still plenty to listen to.
Jellyfin does not organize the music, it’s a way to browse and access it. For a nice client for desktop, look at Feishin.
To actually organize the music, you want something like Picard.









Symfonium is not open source. It is a paid app by a helpful and responsive sole dev.
If you want FOSS, that’s Finamp. But it is not as good. It does also implement offline play features.
The Symfonium dev has responded to and implemented two requests of mine, making me quite happy to pay for and mention their app.