I'm trying to build something similar but for disasters. I'm using meshtastic since everything is open source and mature. So comms is meshtastic, gsm, and starlink for now. User interface would just be phone and hardware wallets so no custom satslink device. Protocol to transmit bitcoin transactions coming from samourai wallet guys. Decent short message protocol pony/txtenna and opensource code. It doesn't do lightning but I suppose that could be easily developed (simpler with cashu maybe). I know that for offline you could go old school and just hand someone a signed transaction (like a bank check) that they can transmit once they have service. But people just aren't as accepting of that these days. Even sats cards have diminished utility in a disaster scenario since they're preloaded with a fixed amount. I think we need transmit and receive confirmation.