"Save everything to the cloud by default" is a standard piece of functionality for mobile apps now, not a small side feature.
Saving things to the cloud as an option, or an officially blessed way to auto-export data locally for your own backup setup, is important though.
All reasonable solutions would require the user to have done their part in setting up a nextcloud instance or something akin to that. And then, as the repo discussed, you would still see "oh, the majority don't know that".
Someone could always fork it and add that feature though!