Before he appeals he needs to get a one year bank statement that says the deposits are from a foreign source. I think there is a document called a credit advice that will show that the source of the funds are from overseas.
If you decide to use the 800,000 baht deposit method then the money needs to have been in the account 2 months before you apply for the yearly extension. Unless you’ve had the 800,000 baht there already you will not be able to meet the 2 month requirement. If you do have to leave and return then the O visa at least doesn’t have the insurance requirement.
This may or may not be an issue but be aware that a wire transfer from the US may not be recorded in your Thai bank account as a foreign transfer. All of my wire transfers from my US bank account to my Thai bank account are coded in my Thai bank statement as domestic transfers because the money from the US goes through an intermediary Thai bank first.
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.