Mumble VS tailscale

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.

Tailscale

I'm using the GitHub version of tailscaled on one of my Macs as a background daemon launched at boot. To upgrade to the latest version, try the following:

Tailscale is a VPN service that makes the devices and applications you own accessible anywhere in the world, securely and effortlessly. It enables encrypted point-to-point connections using the open source WireGuard® protocol, which means only devices on your private network can communicate with each other.

Might be possible to do using a VPN as long as you can get broadcast/multicast packets forwarded.

Tailscale unfortunately doesn't support it...yet?

https://github.com/tailscale/tailscale/issues/1013

Tailscale - Make depaware output patch compatible

Tailscale is another way of doing it. I'm using it to access my Pi's Samba shares from my phone but it works from Windows as well.