I've been pondering similar regarding data persistence. One half-assed idea I have is simply to prompt the user to save the data regularly. A reminder with a button/link which triggers data serialisation from storage and then triggers the save dialogue. The serialisation/save could be triggered manually any time. A reciprocal import would also be needed of course.

Another similarity half-baked plan is for a PWA to regularly post (encrypted?) data to a service I'd host that simply forwards the data to the user's choice of cloud storage. It would require initial setup and probably regularly refreshing token, but should be workable.