I don't know if it would be what you want but I used Plex as my music server and Plexamp as the client. It will take your library of FLAC and when you download to your device convert to mp3 with whatever bitrate you set.
As a suggestion, have you looked into creating your own music server? A while back I took my folder-based music setup and went through and tagged everything. So now I can find things by artist, album, genre. It took a lot of time but it was very worth it and much better than my old setup.
In addition to the Plex/Plexamp server/client setup, I've also used Navidrome/Symfonium, and Jellyfin/Finamp. Whatever I want for offline use I just click "download" in the client and I've got my playlists, album, etc. information still there. It is far better than my old folder structure.