on my Android devices I use the “Send only” option and on the Synology NAS I use the “Receive only” option to have a backup copy of the files of some folders (typically Documents, DCIM etc.).
Syncthing, set with the options described, “Send Only > Receive only” saves (synchronizes) on the NAS only the latest version of the files on Android devices.
Is there a way to set Syncthing to allow you to keep any previous versions of the files? If so, how?