3 3-wide Z1 + 1 hotspare would work nicely, also.
But I figure more of the folks doing the hoarding would tend to prefer having true dual parity across all six disks, rather than individual single parity across two groups of three disks.
Keeping it identical, vdev-wise, with three 3-wide Z1 makes future upgrades easier. One 6-wide Z2 + one 3-wide Z1 makes the most out of what you've got, with no planning for the future.
Pick your poison! =)
