I just did my 20th yearly extension based on retirement with 800000 baht in bank account.Did extension at Chaeng Wattana Bangkok.
All they required was letter from bank stating amount in my bank account and then copies of my bankbook for the last twelve months, so they could
check money transfer from overseas to my account.Also required to see original bankbook with update on day of extension.I never had any problems in the twenty years
I did extension, maybe because my account never goes below 800000.