There's LoRa runs on 450/900 MHz with complete stack for transmitting and relaying messages. There's even modern encrypted protocol (meshstatic) built on it.

Another comment mentioned that as well. TIL. I'll look into this before anything else [2].

Cross-platform bluetooth between Android/iOS is too annoying sadly. If LoRa can help address those issues, it would simplify things a lot.

[1]: https://news.ycombinator.com/item?id=43552585

[2]: https://github.com/nizarmah/igatha/issues/5