To meet the 65,000 baht/month requirement for a 1 year extension as retired the money has to be deposited into your Thai bank account from a foreign source. They don’t care how you got the money. Only that it comes from outside of Thailand because you can’t work here as a retiree and it will say so on your OA visa sticker.
I am not aware of any policy that would cover a pre-existing condition but I don’t have a lot of experience shopping around. I’ve read comments of others who had such policies that mentioned that they were expensive. I would think so. How else is insurance supposed to work as a business model? If it is about risk management then you can’t have people waiting until they are sick to buy insurance. My Thai policy, that I bought at age 62, has a few exclusions.
You will always need insurance if you come here on an OA. Even when the visa expires your extensions will require insurance. You will need insurance for the second year and if you travel you will only be stamped in until the expiry of your insurance. If you retire here using a non-immigrant O visa then you will not need insurance.