Would use off the shelf hardware and displays like Snapdragon. Be sure to implement Trusted Execution Environment and Vault. Would build on top of GrapheneOS to take advantage of their security enhancements. Big issue would be the question of country of manufacturing and costs of one over the cheapest. Could implement a lot of crypto hardware options but would drive unit price up. You could also offer a home server to pair with phone. Then run usual home services to access from phone.

I'd call it SatPhone to confuse the hell out of people.