I hired a tax legal firm this year to do my 2024 US taxes. The firm is familiar with the tax treaty between the US and Thailand. If I was paying taxes in Thailand I would let them handle it.
One strategy in the first year of living here is to move to Thailand after July 1 and bring the 800,000 in during that final 6 months. Less than 180 days and you are not a tax resident for that year.
I had an OA visa (2017-2022) and I have insurance from Pacific Cross. I bought the policy in 2018 when I was 62 years old. I have a 40,000 baht deductible to keep the price down and because routine care in Thailand is so affordable I would prefer to pay out of pocket. I have never made a claim so I can’t report on that but I am getting a substantial no claim discount. As a result the price of the policy not changed much in 8 years. I currently have a 10 year pensioner LTR visa and I am using the Pacific Cross policy to qualify for the LTR.
If you intend to keep your money out of Thai banks you are going to have to do a border run at the end of the first year. You will have to return to your home country after 2 years to apply for a new OA visa. If you travel to your home country frequently then that will be manageable but otherwise it sounds like a lot of trouble and expense.
Even if you are fit accidents do happen here. I originally came here on an OA in 2017. I got Pacific Cross insurance. I’ve never made a claim with them so I can’t say anything about that. I am also using PC insurance for my LTR visa.
I also recommend you come here with an O visa. Insurance though is still a good idea.
I have a Thai insurance policy, Pacific Cross, and at one time they adjusted my coverage by a few months to synchronize my insurance with my OA visa. Pacific Cross also has yearly premiums. You should talk to your insurance company about your needs. You never know . . .