Have you taken a look at Syncthing? I use it to sync the desktop music directory (~300Gb) with the phone's microSD card. For external storage or cloud backups,

Have you taken a look at Syncthing? I use it to sync the desktop music directory (~300Gb) with the phone's microSD card. For external storage or cloud backups, I just use rsync. It's all pretty trivial once it's setup.