Hi Jacek, per the helpful information provided by Todd and then Ellie, my information does not appear to help you. Based on the answers given, it seems that if the embassy of your country can issue an income affidavit, then some of the Thai immigration offices expect this to be provided. As mentioned, you could try the Mae Sai office as a long shot. For the bank statements, I had downloaded them from online and visited my local branch and asked them to stamp each page, which they reluctantly agreed to do. They would prefer for the statements to be ordered from them and then they would stamp - this method has a lead time, I think seven days, and a small cost associated with it. Best of luck to you.
I'm in Chiang Rai and extended my non o retirement visa in March using the 65k monthly income method using bank statements as proof. I do have a British passport. Very strange to hear only certain nationalities qualify for this method. You could try the office in Mae Sai and see what they say. They used to have a better reputation for service and knowledge. I haven't used them recently.
Thanks Todd. For the OA visa I was struggling in my research with how to get a medical certificate in my home town and also I had failed to think through the health insurance policy. Yes, I will want health insurance but it would be for far less than the 3 million baht coverage now being requested. Thanks for making me think that through. The OA is even less appealing to me now.