just to clarify, when I converted my visa exempt entry to a 90 day Non O based on retirement, the only 2 options were THB 800,000 in a Thai bank or a letter from my embassy (if the embassy still processed income verification). The THB 65,000 deposited into my Thai bank account was not accepted.
However, when extending the 90 day Non O, the monthly transfers were accepted.
You can apply for a 90 day Non O visa based on being 50 or above online with the Los Angeles Thai Embassy and once you arrive, extend the Non O for 12 months at Thai Immigrations OR enter visa exempt and convert to a 90 day Non O at Thai immigrations and then extend that visa for one full year. Both ways are not difficult and you can do either.