Syncthing could be used to replicate a directory somewhere, but that doesn't address backing up the phone itself (apps, settings, SMS messages, etc.). Only option I'm aware of is iCloud. You can connect the phone directly to iTunes on a computer and back it up that way, but that only works with a hardwired USB connection and can't be automated, so it's a non-starter for a regular backup system. Android probably has more options, I'm referring to iOS specifically here though.