Vanadium will add more data to the device-to-device backups but we haven't gotten to it yet.

For Signal, you can set up their own backups locally and they'll be included with backed up home directory data if that's enabled. Signal encrypts their database and encrypts the key used for it with the hardware keystore. A generic backup system can't back that up directly. The encrypted database is useless outside of the current app install since the hardware keystore key can't be exported.