Try to get the C of R and the bank account going as soon as possible and start the monthly transfers.
You don't have to leave the 800K in for years. You could use it for your first extension, by the next one you should be caught up with the monthly deposits.
Where are you going? I think (but I am not sure) if you go to a Thai Embassy in your home county, you could get a one-year retirement visa with your home country bank account, by the end of that year you should have a year of transfers.