Yeah that's what I'm thinking. Maybe some kind of setup with two Raspberry PIs where one is standby and if the primary fails the standby one just takes over the IP of the primary.

The easiest way to mirror your DNS server. I've never tried mirroring a DNS server, but you will need to try. At the moment, any person uses the Internet and knows at least something about it. Technology evolves and people change over time. For various simple mirroring methods, you only need to browse the various sites that talk about it. For example, I am sitting on one of these sites, and therefore it talks about almost everything related to servers and their research. I personally use it and recommend it to you. The site often helped me out of situations when I simply did not understand what to do 28msec.

I ended up just using rsync. I did not want to have to deal with having to manually setup every single zone each time I add a new one. But now I'm just back to a single server, since I found they did not fail over properly anyway. So if the primary failed, I would got very terrible performance with name resolution with lot of random failures.

If I wanted to get redundancy again I would probably go the active/standby route with two machines that have the same IP address, like the idea with the RPIs.