You didnt’ mention your boot disk, are you booting from a sata drive? USB is discouraged.
A six drive VDEV in Raid-Z1 is risky due to the 20TB drive size. If a drive dies and your system is resilvering, it stands a good chance of another drive dying. I would recommend backing the data up and recreating the VDEV as Raid-Z2
I’ll just point you to the Resources articles. Browse that to get ideas about ZFS related items. There are a few builds in there. You probably should consider getting a HBA for the drives to attach to as it allows more drives but will take up a PCI-e slot.