GrapheneOS includes an encrypted backup system covering far more data than Google cloud backups. It backs up data for apps opting out of cloud backups with allowBackup="false" since it operates in the device-to-device transfer mode. The backup system supports arbitrary sync services with a compatible API. Backups are per-profile so you can test it by restoring to a secondary user.

We plan to entirely overhaul the backup system but it already works fine. It could be a lot simpler and cleaner both in terms of implementation and user experience. We're in the process of overhauling the other apps first but we'll get to it.