Nice to see it working!

Well that's a shame. But thanks for the extra info.

That's been my experience as well (except the plane bit,that sounds like fun!)

Encryption is almost entirely illegal on the ham bands. You can encrypt control signals to satellites but that's about it.

I can think of two reasons (well, actually three) why 868MHz is used more than 430MHz. Many places that allow both frequencies allow more Effective Radiated Power on 868MHz versus 430MHz. Longer wavelengths are more transparent to things like foliage or structures, but a lower power requirement would all but nullify that one advantage. Antenna size. Antenna size is directly proportional to wavelength, 868 is in the ~33cm range, 430 is ~70cm. Any given antenna design is going to be double the size for 430MHz. A half wave dipole antenna for 868 is going to be about 18cm long, for 430 it'll be about 34cm. Not a problem for stationary nodes like what you picture there, but what about the pocket sized node you pair your phone to? Now you've got an antenna the size of your forearm sticking out of your pocket. Bonus third reason is the societal tautology: More people use 868 because that's what more people use.

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.