day tourist visa before you come to Thailand. That gives you enough time then to open a bank account deposit 800,000 and apply for the non o retirement visa.
most Amphur refuse to do it, which is why people use Bang Rak. I am usually opposed to using agents, but this is a situation that is well worth it or you will spend as much travelling back and forth to Bkk and getting interpreters, witnesses, documents translated etc.