"There are other requirements too for non-oa, like Thai health insurance from specific providers. " . . . . . . no "Thai health insurances" required for the first two years. Foreign health insurances will be accepted
it is both: for the application to the 365-days Non-Imm-O/A Longstay Visa, you need proof of a thorough medical checkup, and a one- or two year health insurance
Non-O/A visa, it needs a police record check, a medical check, a mandatory one or two-years health insurance and a minimum of 25,500 US Dollars (equivalent of 800,000 THB) in your home bank account (it says "bank", not fixed account, or stock market bonds or whatever else!)
The Non-O/A visa is a beast on its own . . . . . . Funds in your home bank only for the first two years. As soon as you switch to the Extension out of a former Non-O/A Visa, you need 800,000 THB in a Thai bank account in your sole name, AND a mandatory tgia-listed Thai private health insurance
unfortunately being early retired does not qualify you for the "retirement visa" and the subsequent 1-year Extension of the Stay Permit based on Retirement. This visa can only get applied for if you are already 50 years old. . . . .you are 40 years old, this leaves the DTV (Digital Thailand Visa) as a 5-year visa option, or a costly (needs to be bought) ELITE Privilege Visa