An alternative to Syncthing is Resilio Sync. I use both, for different purposes.

Resilio is a battery eater on mobile, but it is a bit more consistent than Syncthing (though after 10+ years and probably terabytes of sync, it’s not like ST is problematic).

RS offers Selective Sync, so I don’t have to sync an entire folder, but can pick a file to sync “right now”. This is really useful for my media folder (~2.5 TB), since I don’t have that kind of free space on my phone. From anywhere I can fire up RS on my phone and grab whatever movie I want (or any file from my laptop, other phones etc, because all those files are synced to my server using Syncthing).