backup-server --> rsync + ssh --> source server
rsync -avzh --progress -e "ssh -p 2222 -i /root/ssh_cert/id_ed25519" -r [email protected]:/nextcloud/ ./
source server --> targz + ssh --> backup-server
tar zcvf - /your/directory | ssh -i private.key backup-user@backupserver "cat > /backup/file.tgz"
source: https://serverfault.com/a/984910