you still need a certificate of residency from immigration and they will not issue you one if you are on a visa exempt entry stamp. As the OP didn’t state how long he stays in Thailand per visit and what if any visa he uses it’s impossible to give him the correct advice.
you are on the right track Mary, but it 2 months before your extension and 3 months after your extension that the 800K has to remain in the bank, and you are correct it must not drop below 400K during the rest of the year.
in my opinion also, but I am happy with my Non O based on marriage to a Thai National, have been on it for years, and this year we clock up 37 years of marriage.
I think you mean options, correct. You have only one option if you are on the Non OA that is use one of the recommended insurance companies to purchase your health insurance, as I mentioned above the Non OA doesn’t allow you to self insure.