I should have said, my experience with the MFA was that for them to do their certification, you need the passport certified by the British Embassy first. Having said that, i have seen posts from several people who seem to wander into their local Amphur with virtually no documentation and walk out with a yellow book and pink ID card a few minutes later🤷♂️. As always with Thailand it varies from office to office, but it took me weeks to organise mine.
You can request it online and send your passport to the British Embassy. I did mine that way a few months ago and it came back in about a week. I don't know where you live, but I was lucky as I live in Hua Hin and could access the Ministry of Foreign Affairs here.
so are you saying on the retirement extension, your balance can't go below 800,000 to 400,000 three months after the extension and return to 800,000 two months before the next extension?