If you care that much about power why are you using 8 tiny 2TB HDDs instead of 1 or 2 big HDDs?

You don't need a NAS for 16TB, you just need a RasPi with a 16TB USB HDD connected to it and a second one for backups that you keep mostly offline.

In a RAID6 with 8 drives, you can allow one disk to go offline and remain able to write, or two disks offline and remain able to still read 100% of data. You lose 2 of 8 = 25% capacity on redundancy.

With a mirror of 2 disks, if one disk dies, you can still read; if two disks die, you're toast. And you lose 1 of 2 = 50% of capacity on redundancy.

A quite different balance.

Because TCO is less with these cheap old drives and I can have RAID5 instead of just RAID1 or no RAID at all.

But you're right. In a few years it will become advantageous to switch to a couple of larger HDDs. I could probably do it right now, but I don't yet trust these new drives as much as I trust the old ones, especially since the refurbished scandal.