I already have a DIY NAS w/ 14x 14TB drives in it running ZFS on FreeBSD. It does not play nicely with Time Machine over the network though, and has some other bugbears that I've resolved to fix by migrating to Linux and running ZFS on Linux, but have never got around to doing.

A 2 drive anything is not replacing my existing NAS + solving my backup use case, although I appreciate the sentiment of saving money.