turn off data and roaming and always use on wifi. Only when you HAVE to use it though. Otherwise keep it off. I’m fortunate in that my whole family and most my friends in the US all use iPhone. So I have my Thai sim and can FaceTime and text message to them for no cost.
I would never buy my phone here. I use iPhone and apply pay. Thais can’t use Apple Pay. If I buy a phone here, I can’t use Apple Pay. Thus, I buy in the US. But your phone there and when you get here, get a Thai SIM card for it. Keep your old phone with your US number in case you need it. Been working for me for years. I don’t notice the $60 dollars I spend to keep the US number.
I highly recommend you get a 60 day tourist visa which you can extend for an additional 30 days when going for a retirement visa. I also, highly recommend you do not try to come on a one way ticket without having an exit ticket. I recommend getting a ticket to someplace like Singapore or Vietnam for your exit ticket. Either get one you can cancel or eat the $50-$75. When I presented my one way ticket the airline was going to deny my boarding until I showed them my exit ticket. DON’T chance it.