I just got my first one year extension to my non o (retirement) visa in Chiang Mai last month. Immigration wanted to see the bank letter as you describe but only wanted a six month statement which I obtained at the Bangkok Bank branch at central airport plaza on the spot with no waiting.
You say you are getting a new visa stamp on Friday. Do you mean an extension? If you’re getting an extension at an immigration office on Friday…consider getting your reentry permit at the immigration office at the same time and avoid getting it at the airport.
mine actually did go beyond my thirty day stamp…. Immigration folks told me not to worry because I had the “under consideration” stamp in my passport. The visa finally came about 10 days past the thirty day stamp. Again, this was in Chiang Mai.
Be aware that Chiang Mai frequently takes longer than 15 working days. Just a heads up in case you’re planning to leave the country soon after the day you expect to get it. Mine took almost a month when I applied in November here in CM.