Hi there! I have a somewhat setup right now that is similar to what you ask. I use pure FreeBSD 11.1 instead of FreeNAS, but FreeNAS is FreeBSD so this will apply. You will have no issue mirroring your 2 drives, but remember that you will need to have a full other drive (SSD recommended) for the OS. Nextcloud works on anything that can run the *AMP or *EMP server stacks. I have Nextcloud working flawlessly in a jail on my server. Only thing broken is memory info in the admin panel, and that’s mainly because Nextcloud gather system info from /proc, which FreeBSD doesn’t use, and even if it did, jails don’t get access to it.
I often recommend this tutorial to folks who want to set up Nextcloud on FreeBSD. Just be sure to tweak it to your use case, I never followed it dot-for-dot (like the Let’s Encrypt part)…maybe I will have to create a tutorial too since I’m seeing more and more folks ask about Nextcloud on FreeBSD
EDIT: I would also set up Nextcloud in a fresh jail and not the premade template. It always seems to be a bit behind and lacking in control.