Get a good agent, you trying to navigate it yourself is like peep in US filing their own tax returns, it can be done, but nobody does it. You are getting tons of piecemeal advice and i see their disparate words versus a very clear to path to get what you want with low cost and hassle. i researched a good agent with comprehensive due diligence, once identified, he handled EVERYTHING for me and is my trusted advisor on all visa and entry matters. Find a good resource and stop trying to DIY. Immigration prefers you work with an agent compared to whatever these myriad comments advise.
i just use my US phone in Thailand. my carrier has an allowance for international roaming and I've never gone over it with just 2FA texts. Check if your Carrier has an international roaming allowance or even what the cost is. I use a separate phone for Thai domestic data and calls. The US phone can stay home safe and secure