Does your immigration never verify your money via statement and certification from your bank that it’s your account and money came from outside the country?
Why don’t you explain how you can get a statement showing that you lawfully had the amount in your account over 5 months in each of the last 3 years?
You get the nonO visa from the Thai embassy in your country.
You enter Thailand.
You open an account and send 800k THB (must say “foreign transfer” on the statement).
You go to the immigration and get a one year retirement extension near the end of your visa.
If one branch doesn’t let you open an account, Goto the next. You might need a proof of residence from the immigration. Best chances are Bangkok bank, then Kasikorn.