Interesting, that’s a clever solution!
Yes you can’t keep your Duplicati data in the default location of /root/.config/Duplicati because it gets wiped out during major upgrades. Personally I just made a symlink to a folder in /volume1 - i have to recreate the symlink during major upgrades though. And I still have to start Duplicati manually when my NAS reboots (which is not very often). So I like your approach.
I wonder if DSM Task Scheduler just has a maximum runtime allowed. I notice your task was terminated 5 days and 1 hour after it started. Out of curiosity is it always that timespan? I tried searching for max runtime for DSM Task Scheduler tasks but wasn’t able to find anything.