Both are acceptable for a 6 month stay. I'd advise against the STV, it's significantly more complicated.
Non-OA would be the route that I'd go, if you have the 800,000 baht in your bank accounts. You can buy insurance for only 6 months. Non-oa holders are stamped in for the time of their insurance if its less than 1 year.
Actually you're right, I stand corrected. The need to Thais to have covid insurance was for the Phuket Sandbox. They didn't need covid coverage if they were doing ASQ in Bangkok.
Rebbuttle to the sidenote: The Thai embassy in LA just posted an update today advising people traveling within the 1st week of November to apply for BOTH, a COE and Thailand plus when it becomes available.