What is the best visa option for a 53-year-old moving to Chiang Mai, Thailand?

IMO the best is to get a Non-O at a Thai Embassy/Consulate as the 90 day permission to stay upon entry can be extended for one year for purpose of retirement at Thai Immigration - in this case, the 800k will need to be in a Thai bank OR if using income method, at least a couple of transfers of at least 65k to a Thai Bank (can be done in the first part of the 90 day permission) - 30 days before the 90 day stay concludes, you can apply at Thai Immigration for the one year extension - no health insurance requirement.

If you apply for a Non-O-A, it must be done in your country of residence (usually home country) and you will be required to have health insurance coverage showing documents required by Thai Embassy/Consulate where you apply. Also, be aware, that although a Non-O can be obtained for purpose of retirement at most Thai Embassies/Consulates - that may not be the case in your home country, e.g., in USA, they will not issue a Non-O for purpose of retirement, but insist that you apply for the Non-O-A instead.

Depends on your nationality, that 30days (without visa ) could be visa exempt entry or visa waiver on bilateral agreement.

Another possible option might be convert tourist visa to Non-O (being over 50) inside Thailand and following extension. But not every immigration office issues conversion.

You can apply for the Non Immigrant O-A visa but that must be done in your home country.

Or you can apply for the Non Immigrant O visa and continue with the Extension of Stay based on being over 50 years.