Meshtastic

I think this would have worked better with a meshtastic node hooked up directly to the pi while using the meshtastic python library to interface with that device. You could then display any messages sent to all devices, though it wouldn't help much to grab messages sent to other users since those are encrypted for that particular user. Using the hackrf to decode messages seems like it would be a lot harder, though possibly doable. I don't know how encryption works on meshtastic, since I haven't looked into the messaging protocol at all, but you might be able to find a vulnerability with the encryption that would allow you to see messages meant for different nodes. That's something the hackrf would probably be better at.