What should I do about the additional two months required by Bangkok Bank for my Non-O retirement Visa extension?

The first thing is that you need to make sure you meet the basic requirements for your extension. That means you met last year's requirements. Your 12 month bank statement will have to show that you kept your balance at 800,000 for 3 months after you received the extension, and then never let it drop below 400,000. If you can't show that, you're already done. Nothing else matters, you won't get a new extension.

If you did meet that, and now you've brought your balance back up from 400k to 800k before you will be applying for this extension, then you're going to have sign a document for Bangkok Bank. That letter gives them permission to freeze your 800,000 for 4 months. Once you sign that letter, they will give you the proof you need for immigration. If you don't sign the letter, you don't get the proof. It shouldn't matter much since you already have to keep the money in your account for 3 months after your extension, so the bank will now just be requiring you to keep it an extra month after that.