@Dave ****
not really that strict. If you keep the 800k in your Thai bank account, start transferring in at least 65k each and every month as a transfer from your home country bank. If you've done this for the 12 months prior to your next annual extension of stay, you should be able to change to the income method. Once you're approved for your next Extension of Stay under the income method, you would be free to withdraw/use/transfer the 800k.
ALWAYS confirm with your local immigration office as each can vary.