FreeNAS uses ZFS, and manages all the disks (software RAID). It can do RAID 0, 1, Z (equiv. to 5), Z2 (equiv. to 6).
I'm running XignasNAS which is a branch of the old FreeNAS. I have a boot SSD and then six 3 TB drives in a RAIDZ2 configuration (can lose 2 drives).
Anyhow, you want a separate device to boot off of, either a USB stick or a small drive.
For simplicity, I would suggest RAID 1 mirrors, and adding drives in pairs (to be RAID 10).