Get a Wise card and a Charles Schwab account with a debit card. Charles Schwab will refund all ATM fees at the end of the month...Wise is a great back up just in case you lose your Charles Schwab card. Depending on airport arrival, book your transfer in advance in case they don't offer Grab or Bolt at the airport. Get THB when you land as you may have to pay cash for your transfer (Bolt).
Start taking a strong probiotic now and bring them with you and continue to take them. Bring activated charcoal with you in case you do end up with an upset stomach. That and coconut water will fix it.
Enjoy! It's such a beautiful country with lovely people!