Not sure about linking 2 HAs together, that could get complicated real quick.
If you have your DMZ setup, put HA in that and only allow web access from your main network. Also with camera, there is a feature to allow direct access if you and the camera are on the same network.
If you run a single DNS server, you will always have downtime when it’s restarted.
The only way to mitigate that, is to run 2 DNS servers.
I setup my network to use pihole as the first DNS and the router as the second, most of the time pihole is used. Unless it’s down