fwiw, no sure your use-case but might simply that setup.
no smb, no nfs, no mount.
just a simple rclone copy direct the qnap sftp server.
could run rclone copy on your synology box, not use ubuntu as a middle-man.
on my synboxes, i use rclone copy veeam backup files to cloud.
check out my topic. i rclone copy to a synology sftp server.
https://forum.rclone.org/t/on-some-sftp-servers-e-g-synology-the-paths-are-different-for-ssh-and-sftp-so-the-hashes-cant-be-calculated-properly/36506/1