Does running pi hole make this redundant, or are they solving different problems?

I wish I could upvote you more than once

Doesn’t losing a vdev cause the entire pool to be lost? I guess to your point with sufficient redundancy for new vdev 1 drive redundancy whether 3 disks or 5 is essentially the same risk. If a vdev is added without redundancy that would increase risk of losing the entire pool.

https://archive.org/details/swgrap