dnsproxy-config VS gravity-sync

Yes.

You can find the project here, the documentation describes installation alongside or in front of Pi-hole, depending on the use case. If you want secure incoming traffic, a certified FQDN is required. This is left as an exercise for the user (to raise the bar somewhat).

No need for the full AGH stack. You can easily run dnsproxy alongside or in front of Pi-hole.

I have somewhat documented the process of putting up a Do(H|T|Q)/DNScrypt proxy as a standalone service here.

I still need to fix up a couple of permission issues in my documentation, but I have the rough outline of deploying dnsproxy as a standalone simple service here. The last few edits I made to it were getting it to use the yaml syntaxed config file, and trying to make sure a default configuration should Just Work without having to edit it, but you'd still want to.

There's a basic overview here, but I haven't touched it for a while and the documentation may be slightly broken surrounding exec permissions on some files.

It's very possible. You could easily set this up yourself. I do so so myself using dnsproxy, and you could too.

I have an example dnsproxy service here that I use for high availability (one-to-many) distribution and providing encrypted transport.

You might know me as that Unbound guy, or maybe even that dnsproxy guy. Today I would like to make myself known as that Munin guy, and introduce munin-pihole-plugins for Munin Monitoring.

Kinda.

OR I could implement https://github.com/vmstan/gravity-sync as it's last commit was a year ago but users still seem to be using it.

I run the split DHCP approach for years without issues. Having only one PiHole with DHCP Server doesn’t give you a fully redundant system. I don’t use Gravity Sync myself, but their wiki covers this case with examples: https://github.com/vmstan/gravity-sync/wiki/DHCP-Replication

the github page has a really good walkthrough and explanation

I don't have anything add to your question. But I recommend installing gravity-sync if you're going to run two piholes. It will sync up DNS entries (among other things) automatically to the second pihole when you add a entry one of them. https://github.com/vmstan/gravity-sync

If you go the 2 pihole route, I strongly recommend Gravity-Synch. https://github.com/vmstan/gravity-sync

Run Pihole on both, if DNS goes down its painful. You can sync them with https://github.com/vmstan/gravity-sync