Well, actually, in the end the Heltec ESP32 module could not be made to work with ROS and LoRa simultaneously :( I'm sure it's possible, but I ended up spending too much time on it and decided to go for an Arduino Mega 2560 Mini Pro instead, which worked straight away without any messing about. The end solution is a bit more expensive as there is no onboard LoRa chip, but time is also precious!

It's now time to get another run or PCBs made and incorporate a few more handy features such as the ability to turn the power on/off to the RPis from one of the micro-controllers and get auxillary power via Type C USB cables.