I agree with @Magissia
Duplicati on DSM should install its settings into the proper app location.
In DSM Package Manager, admin can choose where packages should install (even create separate volume on the NAS for package locations). If the package if prepared properly, then it can allow the DSM settings to determine on which volume the app should install (and can even be moved by the admin using DSM).
The data in this location survives upgrades (even if package is upgraded by uploading a new .spk manually - provided the app is packaged properly).