Also, having two Raspberry Pi for primary and secondary dns is good practice, in case something goes wrong with the main one.

Why not run pi-hole in one of those kubernetes cluster for Raspberry Pi, and don't forget a set up a UPS for redundant power supply.

Or: in the rare eventuality that your raspberry pi dies, it takes 15 seconds to open your router interface and reset to the ISP DNS. Work smart, not hard.

I dare you to tell my wife how easy it is. I still remember OpenDNS being blocked in France the exact day I went for business trip and me not anticipating it (I didn't remember it was set in pi-hole)