If you're using Android then there's a sweeeeet free and open source GUI app you can find here¹. I'm using it myself and genuinely prefer it to the default SyncThing GUI.
If you're using iOS there's a commercial client². I don't have iOS myself, so I can't comment on it.
In any case, SyncThing is proving to be excellent for all the things I used third-party commercial sync services for. (Like backing up phone photos to my desktop)
—
¹ https://github.com/Catfriend1/syncthing-android?tab=readme-ov-file#syncthing-fork---a-syncthing-wrapper-for-android
² https://mobiussync.com/