agreed most embassies now refuse to give out confirmation on income letters. I am lucky in that I have a number of passports and with two of them one being Irish, my embassies still do these letters. But yes I agree just put the 800k on your account first. But as I will point out although this is a visa page you will be taxed on remitting those funds to Thailand as of 1st January 2024. Unless you had those savings prior to that date.
I wouldn't bother tbh. I've lived here 6 years. Do not retire to Thailand. Find somewhere else. I'm leaving too. You'll be subject to thai progressive tax system up to 35% of your income.
your special you're an aussie each DTA needs to be look in on an individual basis. For example my tax rate in Zurich is only around 21% but tax tax rates go up to 35%