Full backup always includes the audio files so it is rather big. If you just want to transfer changes on lyrics / playlists and song settings, you can create a database only backup which is very small.
If you delete a song in the song list, the audio file is also deleted. So if you create a new backup after cleaning up your song list the backup will only contain the songs that have not been deleted.
About the 35 gigs: try to empty the trashcan in the Files app. Maybe there are still deleted old backups in the trash.