If you aren't on an iPhone already, switching carriers might be easier.

That's a fair point. I did make some assumptions.

iPhone sos does not allow you to just text random stuff to arbitrary people. It’s emergency only and a preselected “family group”

If you have zero signal, modern iPhone allows you to connect to satellite and text using iMessage. I just used it this week multiple times during massive Pacific Northwest blackout.

Works surprisingly well. You have to be outside and hold iPhone in the specifics position pointed at satellite, it tells you where to turn iPhone to to get signal.

Did they get rid of the limitation of having to setup a special family group in advance?

You can initiate a conversation with anyone while you are connected via satellite. But you do need to set up an emergency contact and/or Family Sharing Group if you want to connect via satellite and receive messages that were sent to you while you were offline.

(I’m referring to the "Messages via Satellite" feature that launched two months ago in iOS 18. This is different from the "Emergency SOS via Satellite" feature that has been around since 2022.)

That is not true. If you don't know, don't guess.

https://support.apple.com/en-us/120930