@Rainer *************
, you have extremely limited options.
As
@Mar***
correctly stated is; bank 800K baht in a thai bank account in your name only for 2 months before you apply for a yearly extension based on being over 50 at the immigiration office
That meant in order to NOT run on overstay (and be fined 500 baht a day until your money is seasoned and you can apply for the extension ) you needed the money in the account on July 26th (July 26 - Sept 26 = 2months)
OR
You need to "prepare to depart" the country on or by the 26th of Sept when the amnesty runs out.
I am making no judgement on you or your situation BUT if the truth be told; you've been IN the country since the end of March (almost five MONTHS, you should have started looking into the options you had a while ago.
As I see it those are your choices..