No, not at all.
Set MiaB user user-data to allow logins. Don’t ask me how, I use webmin to accomplish this… and set a password for this user.
I do not use PKA, so if you do, turn it off temporarily in /etc/sshd_config so that you can log in to copy your PKA public key over with ssh-copy-id.
Then log in with the user user-data@box.mydomain.tld to do your rsync.
Easy. If you understand what I just said … if not … well then maybe rsync isn’t for whomever is reading this.