Noah517:
The LoRa telemetry system is a good idea but I don’t see how even with a high gain antenna, you can get 250-400 km with 10 mW.
No need for fancy antennas, 1/4 wave wire with radials on the balloon transmitter and say a 5dB gain omni on the receiver is good enough.
The current record with that sort of equipment is 702km, 868Mhz @ 25mW;