The 800k minimum must be in your account 2 months before you apply for your next extension then stay there for another 3 months after your extension is granted. Then you can withdraw some funds but the balance must be a minimum of 400k for the rest of the 12 month extension. If you use the 800k for your next extension then again it needs to back at 800k two months before you apply.
Thai Immigration under the Ministry of the Interior only issue Visas from inside Thailand, outside it is the Embassies who come under the Ministry of Foreign Affairs, two different government bodies.