This is very helpful. I was thinking along the same lines of getting a phone with dual SIM cards, the iPhone SE, to use when I'm there (in Argentina) and get their local SIM card that I would place in the second slot, but was not sure how it would work. My provider is T-Mobile, but since you say that the only solution has been using Google Fi, I'm wondering whether I should switch to be able to get the codes or was your point that as long as you have a US provider you're covered and able to receive the codes? Please let me know.
Your Movistar sim solution for the bundled services sounds great.