For anyone needing a very basic manual backup to the main array, this is what I use:
Download the NerdTools and the User Scripts plugin
Go to Settings -> NerdTools and check the rclone package. Click apply
Create a new User Script in Settings -> User Scripts and modify that script according to your needs:
#!/bin/bash
rclone copy /mnt/user/<origin-share>/ /mnt/user/<backup-share> --backup-dir /mnt/user/<backup-share>/old-$(date -Is)
Set it to run daily (or whatever you prefer
NOTE: You can omit the --backup-dir option. More info: https://rclone.org/docs/#backup-dir-dir