I have looked at DTA between UK and Thailand that was signed in 1981 - it is extremely vague, does not talk about pension income, or variance between respective countries tax allowances. 40 years ago pensioner mobility was virtually unheard of, so unless there are plans to update the many treaties Thailand has in place, chaos looms next year.
it’s a good question and best asked to a tax specialist who can explain exactly how the DTA works, if the premise is that you must pay tax somewhere, are exemptions in your home country applicable in Thailand? The other element to this of course is personal allowances, they are much lower in Thailand than UK for example so will tax free remittances to Thailand be taxable here due to the lower tax threshold ?
If you are in Thailand for more than 180 days in calendar year (effective Jan 2024) you are a tax resident, and as such required to file a tax return - Double Tax Treaties exist for many countries incl Australia, US and UK, but this doesn’t mean you can ignore the requirement to file. How this will be enforced remains to be seen, but back taxes and penalties will not be pleasant. Folks on LTR visa are exempt.
IDP very important if you plan to drive here, makes application for local driving license easier, you can use your US driving license for 3 months after arrival after which you are not legal.
Only a matter of time before Cambodia (and others) catch up and tax foreign residents, it’s a step from developing to developed if you pay tax in your home country nothing to worry about. Death and taxes as the saying goes.