Instead of rerouting each IP address individually, you could consider to block outbound DNS traffic in your Fritzbox for all or for a selection of your devices (a while ago, I've explained that in more detail in German, see Smarthome Steckdosen im pihole - #8 by Bucking_Horn).
That would block attempts by any device in your network to access any public DNS server via port 53 DNS, leaving Pi-hole and your Fritzbox as the only operational DNS servers.
It would stop all DNS bypass attempts to public servers without having to know any IP addresses upfront, but may incapacitate those devices that insist on using a public DNS server.