gravity-sync VS Mumble

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

I think Sonobus is overkill. I suggest you look at a couple of relatively old-school gamer voice chat tools - Mumble or Teamspeak. Mumble is open-source and the connection is always encrypted, Teamspeak is commercial but the free tier should be fine for you - but you have to make sure to manually turn encryption on yourself. It has been a long time since I used either, so I don't know which is easier. Both of them require you to run their matching server software.

Mumble's latency is unbeatable imo, it's basically their main focus and shows.

The sticking point for me is the lack of persistent messages, something the devs strangely think is a privacy plus. Issue open since 2016: https://github.com/mumble-voip/mumble/issues/2560

If you drop out for a minute you won't have access to anything that was posted in chat, which makes it useless for anything other than voice only comms, that might suit some business purposes but I've always needed to post links or screenshots in chat during meetings.

There's any number of alternative chat applications available, like Element, Mumble, Teamspeak etc.

Mumble?

I think Mumble might fit what you're looking for. It's been a very long time since I've used it, but it seems to still exist: https://www.mumble.info/ - I've used previously for exactly what you're describing, events with lots of crew dispersed around and no budget for radios. I had it installed on an AP running OpenWRT so it was just a case of plugging that in and getting people to install the app and connect to it.

Along with the options already mentioned, if you're not into TeamSpeak, there is an open source alternative called Mumble which operates in the same manner. No internet required, and is supported on multiple platforms.