As per
@Bra****
in short.
Go to your local immigration and get the information on 1yr extension.
Get the 400k in your bank.
Then when you are near the end of the 90 day visa, apply for a 60 day visit Thai family extension, giving your money the time to season before the 1yr extension.
This really is the only path available other than maybe the agent route.
Which is problematic in the long run.