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

It is harder and harder to get non-O's from the neighboring thai consulates based on being over 50.

I'd say get a single entry tourist visa while you're in Vietnam from the thai consulate there. That will stamp you into thailand for 60 days. Then once you're here go immediately to the thai immigration office in Chiang Mai and apply for an in country 90 day Non-O visa based on being over 50 (retirement). You need at least 21 days left on your current entry stamp to do it because the application goes under consideration for 20 days before you go back and get the Non-O visa and a 90 day entry stamp inked in.

Then you wait until you have 45 days or less left on that stamp before you can apply for a year extension based on retirement by meeting the requirements.

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.