@Bruce *******
we have the basic plan on Google Fi which is the cheapest. When you install your local/Thai SIM, you just change "data" to that SIM. For texts, obviously you want to choose the Google SIM. For calls, we always select "ask every time" as you can use your local number for local calls and Google for WiFi calling.
If/when you travel back to the States, you just switch sims to Google for everything.
We haven't had any issues in over 3+ years.