Although the immigration office in Bangkok might not check your bank balance, there is always a chance that the people that are accepting your visa extension could want to check on your account before processing. I don’t live in Bangkok but am on a non o marriage extension and all paperwork is sent to Bangkok for processing, by my local immigration office. I have been in this extension now for the past 20 years and occasionally, I’ll get a call from Bangkok just before the 30 days under consideration is up, to ask me to send another copy of my bank book showing the 400k. Doesn’t happen much, it’s happened to me just twice, but it still can happen. If you can afford to leave the money in there until you get your extension stamped, then my advice is that you do just that.