Pi Hole for LAN ad blocking - Fritzbox Configuration

After seeing it mentioned on another thread I've set up Pi Hole on a R.Pi 1b - one of the early ones. I like the idea of LAN wide ad blocking, plus I guess a cache is good too. Setting it up was easy - an hour while I was doing something else.

It's sitting attached to an old switch in my office, using an old USB power supply. Because I don't think it's very reliable where it is I'd like to configure it in a way that if it gets unplugged DNS still works on my network. If it works out well I'll get another switch for my network cupboard and a better power supply and put it in there, so it should be more reliable.

The Fritzbox only issues one DNS server with DHCP, so I need another way.

Standard Pi Hole Setup

I guess the standard setup would be for the router to give out the Pi Hole IP for DNS in DHCP. I tried it - worked well - until I unplugged the Pi.

My Pi Hole Setup - to Validate

I've set up the Fritz so that the first DNS server to use is the Pi Hole, and the second is the 2Degrees secondary DNS server. The Pi Hole uses the 2degrees DNS, in case they have local media caches. I could easily use Google or CloudFlare, it's easy to switch later.

I figure this way DHCP tells clients to talk to the Fritzbox for DNS. When the Fritz gets a DNS request it first tries the Pi Hole, but if it doesn't get a reply it'll time out and go back to the 2 degrees DNS.

Question

Is this a good / viable setup? Is there a better way to use the Pi Hole as primary DNS but have it fail over to ISP DNS if something goes wrong?

Fritzbox Setup