That which is monthly must come from abroad. If the diff between monthly and savings is in Thai bank , it must remain there, under same conditions as would the full 800k, or 400k married.
you dont need an Agent..throwing $ away. You have funds deposited, at Every Immigration office there are volunteers to assist, they are excellent.
It's not a difficult process, I suggest after 1st year switch to mo income method...very easy, and not leaving any large funds in a bank with no interest.....best of luck.
It really is easy, and the learning curve will be invaluable for your future.