I recommend that you think about what you want to achieve. You wrote that you want “off-site backup” and this is a Syncthing forum.
As you already know, Syncthing is a file synchronization tool which will synchronize one or more folders on two or more devices. So in the event of accidental or malicious deletes/edits on one device, your synchronized copies on other devices will also be updated. This is why the synchronized copies can not be regarded as backups.
Admittedly, Syncthing has a versioning feature which can be seen as a “poor man’s backup”, but for it to be effective you must enable it on a device which is never used in your day-to-day activities.
Conclusion: I think you should use a dedicated backup program instead. Maybe restic.
I intend to have regular snapshot of my backup. So if something happen, I can revert to some older snapshot… Go back in time
I just need now to decide if for that off-site backup, if I will make all my device sync through syncthing to the SYNOLOGY device or simply use the SYNOLOGY app on each of my device
And continue using syncthing between my 3 pcs, mobile phone etc
I do not use Synology, but hopefully you will find it useful anyway. If not, then someone with Synology experience will hopefully be able to offer more specific advice.
As long as you are aware about the difference between “backup” and “synchronized copy” and implement accordingly, you should be fine. Good luck!