so just to update my results here - i tried a hetzner storagebox, but the cifs storage mount was having some stability issues. and it was so painfully slow as to be practically unusable.
so i switched to another option that i've seen mentioned in the forums here, which is to backup to a second cloudron using minio. and i'm happy to report that it's working fine. using an alphavps storage vps it's actually even cheaper, and it's quite convenient to have a further backup if needed.