I'm currently thinkering of building a balloon with a 2.4GHz LoRa transmitter (SX128x) and a low-power STM32U microcontroller.

Why?

- You can repurpose 2.4GHz Wifi gear opening many doors

- You can easily include volunteers dumping data from HF into a IP sink for telemetry. TTGO offers boards with 2.4GHz LoRa.

- Theoretically you still can add a "low rate" 868MHz/433MHz and a "high rate" 2.4GHz for transmitting pictures and other stuff more quickly.

- BOM friendly. As the balloon might get lost you have to plan a bit for costs.