You could use use Rclone for a one way sync. Rclone supports Nextcloud / WebDAV natively and does exactly what you want. https://rclone.org/webdav/
The question remains why you need it. If backup is the goal, there is more to it than a simple one-way sync, and this shouldn’t probably be your only backup solution, because, let’s assume for a moment that you really only use Nextcloud as a target for the one-way sync and don’t change anything on these files directly on the server. You could still loose files, if you accidentally delete something on the client side and only notice it after the next sync has has been executed.