Best way to backing up LXD container running Nextcloud to external drive

I am looking for a fast way to perform regular updates of a LXD container running Nextcloud with ~400GB data.

I cannot store backup data on the same disk (no space) but have mounted an additional drive with 500GB that I want to hold the data.
I guess creating a snapshot is not working, since it is stored on the same pool?

I am running LXD with ZFS.

Thank you for your thoughts!

I would suggest to perform a backup of the content of the Nextcloud installation, instead of the container itself. As described at