One issue I've found with a similar approach is that many mobile apps are limited to iCloud, Dropbox and Google Driver for state synchronization. Only a few apps (e.g. https://2doapp.com, https://notebooksapp.com, https://www.omnigroup.com/omnifocus/) make the effort to support open standards like WebDAV and CalDAV.
iOS now supports storage providers, so you can use a dedicated application to transfer files between devices via SSH/SMB/etc, but many apps cannot export/import all state via a single backup file.
> Any sensitive data is encrypted
Is this done on clients (E2E) or server? If the server, how do you manage encryption keys?