Can’t you just change the reason for your extension the next time you apply?
If you need to do it sooner, and you don’t have a multiple re-entry permit then you can just leave the country and your extension is void. You can return to Thailand on a visa exempt and apply for an O inside Thailand or get one out the country. Different rules apply.
If you intend to stay on a non-OA then the next time you apply in your home country you will either need Thai insurance or have your outside insurance company sign the Thai government affidavit. If you apply for a 1 year extension of stay based on your non-OA then you will need a Thai insurance. Robert answered what might happen to you currently.
The bank letter states that you have an account at that bank and the balance at the time of the letter. Depending on the immigration office that may have to be same day but it shouldn’t be more than an hour at your bank.
If you are using the 800,000k method you should only need 3 months of bank statements but maybe CM wants more.
If you are using the monthly income method then they will definitely want a years worth of statements and that may take a week at your bank.
Finally they may want to see a same day transaction in your bank book
Andy You can use money in your home country for an O-A visa but the O-A has an insurance requirement. There are several ways to get an O visa which is a 90 day visa with no insurance requirement. Eventually getting 1 year extensions of stay will require money in a Thai bank.