I have an asus router with a pi-hole on the network.
I was doing some work on my server and noticed that when pi-hole was down, I couldn't access the internet. I was looking for some ideas online how to deal with this, but they said to have a second pihole on the network in case one is offline. Is that the only way to do it? Is there any way to have the network go back to normal if the pihole is offline?
You mentioned you have an Asus router. Which one?
Why not move to hosting your stuff on the router?
https://www.snbforums.com/forums/asuswrt-merlin.42/
Sure it doesn't completely solve the issue but in my experience it's incredibly stable, and more so people expect to restart the router if the Internet isn't working which simplifies things too.
Also beneficial is that you can give different clients different DNS servers comfortably.
Specifically, check out https://diversion.ch/ for dns blocking but its capable of a lot more.