I'm still not sure what setup I have. I really should have written down every decision I've made while building my NAS (I think I've now found articles that disagree with every single choice I made)

    $ zpool status
  pool: BoxODisks
 state: ONLINE
      scan: scrub repaired 0 in 9h20m with 0 errors on Sun Jul 31 01:31:31 2016
    config:

        NAME                                            STATE     READ WRITE CKSUM
        BoxODisks                                       ONLINE       0     0     0
          mirror-0                                      ONLINE       0     0     0
            gptid/18059e22-b6a4-11e5-9cca-0cc47a6bbf34  ONLINE       0     0     0
            gptid/194649b1-b6a4-11e5-9cca-0cc47a6bbf34  ONLINE       0     0     0
          mirror-1                                      ONLINE       0     0     0
            gptid/1a86b3cc-b6a4-11e5-9cca-0cc47a6bbf34  ONLINE       0     0     0
            gptid/1bcd3ad6-b6a4-11e5-9cca-0cc47a6bbf34  ONLINE       0     0     0
        cache
          diskid/DISK-S24ZNWAG903847Lp1                 ONLINE       0     0     0

I think I have mirrored vdevs.

What does that mean for replacing failed disk/s? What does that mean when I want to upgrade to 8TB drives? The biggest "mistake" I made was buying a very expensive 4 bay enclosure - 4 drives is not enough.

When you want to upgrade, you can offline and replace each disk, then resilver, in turn. If the pool is set to autoexpand, you'll eventually get a total capacity of 16TB. You can do this for all the disks, or for each mirror separately in two groups of two.