@Andreas ********
you can do the following...
Just whack 400,000 in immediately you have your account set up. Then you have 9 months to top it up to the 800,000 required for retirement "extension" or if "marriage extension" you have the 400,000 in their already 😁
Or if you want to use an agent again then the cost for them to 'fast track' you as you have the money in situ the couple of months before. Which I think is around 7,000 baht
Or the monthly income road, bit more paper work but still doable