best description I have ever read of syncthing. they should put this in their readme or about page. describes the pros & cons in an honest way. I've had all the problems you describe and will probably have them again in the future, be confounded, be frustrated.
I get (mostly) worry-free backups
except this; it's sync not back up. ;) but it is very backup-like and in some situations does the job as well or better than backups
I <3 syncthing nevertheless
borg is fab; i don't think there is a mobile component tho?
restic iirc is complicated and more advanced
don't forget rsync which is sync but is the basis of various back up tools