sounds like you need to consult with a thai tax attorney if you want more accurate and detailed information. I was simply helping people understand how often their Thai tax liability will be higher than their Western liability creating a net liability to Thailand
so if you do the calculation on $50,000 of annual income the tax rate in the United States would be 12.5% in the tax rates in Thailand would be 16.5%. so you would owe 4% to the government of Thailand which is the net difference. Makes sense?
the top rate in Thailand is 35%, but the real problem for people is that the brackets are much lower so you'll have a higher percentage rate for a given amount of income. Look at the table demonstrated above and convert the Thai baht into US dollars and then look how quickly you get to a 20% tax rate
assuming you have the 800k baht in a bank account for the required time, you can just leave, re-enter on another visa exempt entry, then go to immigration and apply for the non-o for retirement.