> An important note is that you CANNOT add extra disks to an existing VDEV.

Not true for mirror vdev, but you don’t get the added capacity. You may add vdevs later, however.

Also some good arguments are made for using mirror vdevs over RAID-Z [1]. The extra space afforded by a 1:3 RAIDZ compared to 1:2 mirror isn’t much. Given the performance gain over a mirror resilvering and reduced recovery time, I prefer mirroring.

[1] https://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs...