If you plan to live in Thailand for a long time then might be worthwhile to come on Tourist Visa, open a Thai bank account, fund that with 800,000 baht, convert to a Non-O visa in Thailand at the Thai Immigration office for about 2,000 baht. Toward the end of the 90-day permission to stay from that visa apply for a retirement extension showing 800,000 baht seasoned in a bank for at least 2 months. From that extension, costing 1,900 baht you will receive permission to stay for one year, which can be reapplied for in subsequent years. You are then required to keep at least an 800,000 baht balance in the account for two additional months and for the remainder of the year the minimum balance of 400,000 must be maintained.
The reason to do it the above way is that you free yourself from an obligation to have general health insurance from a Thai insurance company that has been approved by Thai Immigration. If you come on a Non-OA you will be obligated to fulfill this requirement each year you get a retirement extension based on this original Non-OA.
The advantage of a Non-OA is that you can avoid having to tie up money in a Thai bank account for at least a year or possibly two. But to get that extra year of permission to stay you would need to leave and re-enter Thailand which might require quarantine (as it currently does). Also, to continue with the Non-OA in the future you would need to leave Thailand and return to the US to obtain your next Non-OA visa. In pre-covid times leaving and entering countries was much less problematic. The disadvantage of a Non-OA is that you are tied to the health insurance requirement and there's no way to escape it other than what I outlined in the first paragraph.
I realize do-it-yourself is not everyone's cup of tea, but you can use TurboTax (and other similar) to file US Federal Income tax return for free from this site if your Adjusted Gross Income (AGI) is less than $72,000. The simpler your tax situation the easier it is. ;-)
being on a retirement visa or extension of stay means you cannot work in Thailand but does not mean you cannot earn income in Thailand.
In fact, many people with an extension of stay for retirement in Thailand keep money in a Thai bank in excess of 800,000 baht which earns interest. That interest is reportable both to the IRS and Thai revenue department as income. If it's below the filling threshold you may not have to file a Thai income tax return, but by doing so you may be able to recover any tax withheld from the interest income you received. That interest income is reportable as a tax liability on your US Federal Income Tax Return (if you are required to file). You may be able to claim the Foreign Income Tax credit on that, but you will not be able to claim a Foreign Earned Income Exemption (FEIE) because interest income is specifically excluded from that.
The alternative is for me to use my Thai address and have the mail sent to me in Thailand. There are three problems with that alternative:
1) While mail delivery is generally OK in Thailand if you are unfortunate enough to have your letter misdelivered to the wrong address it may just languish there forever (the discipline required to return mail to the post office if it has been misdelivered in Thailand appears to be an underdeveloped skill 😉 ).
2) I don't want my US financial institution to find out that I live overseas and want to restrict or close my account. There have been reports of this happening. For example, it is generally not possible for a US company to sell mutual funds to someone who doesn't reside in the US for legal reasons. I don't want to be encumbered with such a restriction. I regularly buy US mutual funds and I live outside the US.
3) I want to be able to open new accounts in the US and this often requires that I have a US address. Note: businesses if they are interested can determine that what I have is a CMRA rather than a "real" residential address, but I have been able to open accounts (for example, Charles Schwab Checking Account) with only my CMRA.