FreeFileSync a FOSS backup and folder synchronizer, a must have.
FreeFileSync is a folder comparison and synchronization software that creates and manages backup copies of all your important files. Instead of copying every file every time, FreeFileSync determines the differences between a source and a target folder and transfers only the minimum amount of data needed. FreeFileSync is Open Source software, available for Windows, macOS, and Linux.
Yes, Syncthing needs 2 diferents devices to sync, FreeFileSync syncronize whatever you want within 1 device, for example between 2 Disks or from a network disk to your computer disk.
Syncthing works great if there are 2 devices turned on and connected to a LAN or Internet, and FreeFileSync is totally disconected from the internet and only needs 1 device on (the one that execute it)
I use both, Syncthing to autosync all my devices and FreeFileSync to backup all those files to various disconected hard drives