Funny enough, the Pi Zero uses the CPU from the 3 and the Zero 2 uses the CPU from the 3+, so they’re both more powerful than a 2 anyway :)

Just be sure that the second server in the list is also a black hole. If you don’t, all black holed requests will fallback to the second DNS… which, if it doesn’t also black hole them, will wind up serving you ads and defeating the point!

Personally I find a single Pi is just fine for DNS. It only takes like 10 seconds to reboot. Less, if you use M.2 storage via a HAT or boot from USB! That’s pretty fine downtime. But if you’re afraid you’ll knock over the network and get yelled at by your family or housemates, best to use a backup :)