But do look up your country's DTA with Thailand, to be sure of your position. For example, many countries had all pension income excluded from Thai tax. But the UK only agreed to exclude government pensions, for some odd (infuriating!) reason
You don't need the 800k in a Thai bank to get a 90-day non-O from the Embassy in your home country. On arrival in Thailand you can then open a bank account and deposit the 800k, which must be there 2 months before you apply for 12-month extension of stay. Your nearest Immigration Office will issue a Letter of Residency, which the banks require as proof of your address. To get the letter of Residency, you need to take your passport with visa and e.g. condo rental agreement, to Immigration.
Staying 180 days in a year means you become a tax resident in Thailand. You have to submit a tax return showing income you brought into Thailand. (A proposal not yet passed into law is that you would have to declare all income globally, even if you didn't bring it to Thailand). You assess income tax according to Thai law to determine tax payable. Then deduct tax paid in your home country to determine if any tax is due in Thailand.
Remember it's cumulative days in the year, Lee. If you leave on day 179 of (say) 2025 and returned on 31 December, you would become tax resident the moment you enter Thailand
Indeed they do. Most countries protected their citizens' pensions from the risk of additional taxation, but the UK only protected government pensions 🤨