generally you cannot open a bank account on a tourist visa. But if you apply and receive a non-o for retirement 90 days visa in country. Then you should be able to open the required bank account ( bring a passport with non-o visa in it and a second ID from you originating country)
Admittedly the banks can still be difficult and have their own unique rules.
Thailand is trying to insure that you will not tap into the countries social services and become an expense to the country. Especially since you probably will not have medical insurance either
the agent will put the 800k in an account get a letter from the bank. And have you transfer the 800k back to them within minutes.
Get your cash assembled and renew with your funds the next year with your 800k which can then be left in the account to make you legal for the 3rd year
👍 If starting with the non-o visa and then extending. The initial deposit in a Thai bank must be a minimum of 400K baht for marriage visa/extension or 800K baht for retirement visa/extension