One RAID-Z2 offers more protection than multiple RAID-Z. It's much the same with mirroring: you have ONE guaranteed failure that you can withstand without total dataloss, and POSSIBLY more disks that can fail, if they reside in another mirror pair. But that is no guarantee, so both mirroring and multiple RAID-Z allow one failure guaranteed.

RAID-Z2, however, guarantees 2 failures, so in my opinion offers better protection than both mirroring and multiple RAID-Z.

Consider the multiple RAID-Z scenario where you have one hardware failure, combined with unreadable sector on another member disk during the rebuild process. If that bad sector stays unrecoverable, you've lost one file. With RAID-Z2, you can still access the file and doing so will cause ZFS to fix the bad sector automatically.

So you can see what lot's of redundancy is good for; it allows ZFS to keep self-healing everything it touches. And with a scrub you force ZFS to touch everything.

Back to your proposal. I'm not really sure why you would want 500GB or even 1TB disks. Why not 2TB disks instead? Those offer more gigabyte per dollar/euro invested.

You could also buy two new Hitachi 3TB disks, and use them in a mirror. Then in the future you buy 4 more disks, and create a 6-disk RAID-Z2. You can create a degraded RAID-Z2 with just the 4 new disks, and transfer data from your mirror to RAID-Z2. Then destroy the mirror and add the 2 disks to the RAID-Z2, changing its status from DEGRADED to ONLINE.