For 5 week stay, I used money supermarket. Clicked "preexisting conditions" filled in the on line info and it gave me several to chose from. I'm not sure for longer stays but a Thai policy might be better.
If your embassy supplies the affidavit on the 65k I think you can.
Otherwise I think you'd have to use the 800k for first year extension whilst also transferring the 65k in monthly. Then switch the second year to the income method. Think I've said that right. 🙄