I don’t have a QNAP, so missed right section on first try, as it’s only there if one expands the
“Configuring FTP Server Settings” section. Regardless, I’m still not clear what exactly it does, because it talks only about folders, and not about files in the folders. If it’s the same, then the suggestion about rclone won’t work, but you’ll be better protected against malware overwrite.
Do you happen to know what filesystem you have? One user uses ZFS snapshots to protect.
Ransomware protection with chattr cites that, and also suggests a sticky-bit-based approach.