Don't write long messages in an emergency. If it can't fit in one message, it probably doesn't matter. Instead of trying to predict everything, allow communication. Let them figure it out, don't command them.
Also, you might consider using existing messaging app. Then you can interact if have them install the app. Matrix might work. I think Matrix can do SMS.
It makes much more sense outside the US than to pay for a text bundle. My phone plan includes 0 SMS messages, and I don't know what was the last time when I've had to send one was. It's only useful when someone doesn't have internet, which only happens once every few months at most.
You are generalizing quite a bit here, not everywhere "outside the US" is the same. Here almost all phone plans that aren't data only come with unlimited SMS - and we never paid for receiving them anyway.
Also, you might consider using existing messaging app. Then you can interact if have them install the app. Matrix might work. I think Matrix can do SMS.