What to do with extra HDDs

Hey guys, I'm setting up my NAS (openmediavault) and very much enjoying it! It now runs my Nextcloud and a couple of services. I got a mirror ZFS setup of two 8TB drives.

I got another two 8TB drives and am doubting whether I should add them as an extra mirror vdev, or create a new pool for extra backup. I'm not sure if that extra backup is necessary though, since I got a cloud backup already every day. My drives are only used 14% so I'm not even sure if I should already put them in the pool. What do you guys think?