I think we're confusing apples with oranges here. First the 800k will earn interest in a savings deposit and be liable for WHT in thailand. That's is a percentage of the interest not the entire interest - no where near it. So the 800k will attract interest minus WHT which will still leave a positive Interest amount on your 800k which will accrue over time. So you 800k will never go down or be depleted by tax. Income tax is something completely different. If you pay income tax on your earnings abroad or pensions abroad you will not be subject to thai income taxes in thailand. In fact as foreign taxes are generally a larger portion percentage wise than of the thai tax rate you are safe from thai taxes. The whole reason thailand wants to introduce taxes on foreigners (but mostly directed at thais with offshore earnings) Is that thailand wants to be part of the OECD. Countries of the OECD don't want their citizens having a free ride on taxes everywhere. Someone somewhere is going to tax you.
I do believe the only way around the 800k requirement is to have your embassy here in thailand confirm your income in a formal letter. They'll need back statements etc, plus a host of other forms. I looked into it once admittedly a long time ago but my income far supercedes the 50k or whatever it is you need to prove you can stay here. Maybe you could confirm if that is still the case
If you download Netflix in the USA or UK on your phone or computer you have the exact same in thailand. If you want to watch on a big screen just but a smart TV and you can flip from phone or computer to TV.
Get health insurance anyway. It's stupid not to have any health insurance at any time. It's called accidents and emergencies. Or ignore me and walk into flowing traffic the give me a call.
I flew in recently on a one way ticket and got a 30 day visa. I would however show any onward flight at airport immigration but not sure it is necessary. Of course I did a one way as I'm planning to stay forever. Was just travelling when my non o ran out.