I bought Thai insurance, Pacific Cross, when I was 62. If you buy health insurance late in life the prices will be higher and there will be exclusions of existing conditions. I bought a 10 million baht policy with a 40,000 baht deductible. Essentially, I am self insuring for the everyday stuff and I have insurance in case something truly terrible (short of death) happens. A large deductible will lower the cost. Also, be aware that many insurance policies will not cover you well in the US so you may have to buy travel insurance if you return to the US for a visit. The OA visa requires insurance. I recommend not getting an OA but pursuing an O visa that doesn't have the insurance requirement tied to the visa and extensions.
If you are over 50 years old you might want to consider the 10 year pensioner LTR visa offered by the Thailand Board of Investment. It costs 50,000 baht for 10 years but you must have a passive income of $80,000/year. It also has a health insurance requirement. There is no 90 day reporting requirement and there are no banking requirements.
You cannot enter Thailand more than 2x in one calendar year using a land border crossing. You can technically enter by air many times. Two weeks every month might become a problem after maybe 6 months.
I am expecting that I will have to reapply in full in 5 years. I plan on keeping copies of my tax returns and insurance certificates for the next 5 years in case they ask whether I continued to meet the requirements.
I had an OA for six years. You have to show that you met the financial requirements throughout the year, for example with a yearlong bank statement, in order to qualify for a subsequent extension. I am assuming the same logic will apply to the 2nd 5 years of my LTR.
I didn’t ask the BOI directly what they would do in five years because I assumed that they don’t know exactly themselves. I did ask if they would check about my insurance coverage each year and they said they would not. My pension won’t change so I didn’t have to ask about that.