If you are fit and mobile you realy should try to get your extension yourself. It's not just the money you will save but also the multitude of issues that could cause you problems later on. For example if you get a new passport your local office will not transfer your stamp. You will have to go to the office that issued your stamp. It realy is not duffucult and if you present yourself well at immigration the officers should be very helpful. It would help you to get what is required from your bank if you get an IO to write out in Thai what they want to see.
I am not sure what your problem is. Bank statements for 12 months showing a transfer from abroad of
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baht or more is what is required. You might also need to show statements from the bank the transfers originated from. I'm not sure who is refusing to give your what.
I have just changed to monthly payments. I was not asked for anything from my Thai bank. I printed off statements from the banking app and highlighted the transfers. I also had to provide my uk bank statements to prove where the money had come from. My Thai bank book was up dated on the day and also showed all 13 transfers. I had my Wise transfer notes at the ready too but they were not required. I got the extension with very little hassle. Bangkok Bank.