Can't really comment on that. There are too many variables here. E.g. if you are on a SMART or LTR visa, you would have a special tax rate. Also, taxable income does not equal salary.
I am not sure where the 20% is coming from. In general, in Thailand income tax is a progression in stages between 0% and 35% depending on the taxable income.
That would be 180 days per (tax / calendar) year. But yes, a valid concern. And an even mild one if only limited to extensions. Could in theory also be applied for preventing people from leaving the country. 😉 But that is theory.
We are indeed missing quite a bit still when it comes to fully understanding the implications of the new interpretation and the new proposals with regards to Thai taxation. Also, there is no general answer to fit all, it will likely depend on many individual constellations. But if you live the majority on the year in Thailand, becoming a tax-resident here, I would assume that Thailand has the first right / priority to tax you. That however should lower or null many taxes you are due to pay in other countries (merely based on citizenship). I will dive into that deeper, when the need arises.
An "age discrimination visa"? Unthinkable in Thailand!!! 😉 All I say is 50+ years: have it easy to come and stay indefinitely for decades now. All the rest (younger), no way - up until recently.
I was aware that the US follows a world income tax logic. But I don't know enough about the details. I would assume that there are ways to subtract taxes paid in another country / your new country of residence from what you would owe in the US as a citizen.
Gotcha. Always good and essential to be prepared in tax and financial matters. I am not a expert of US taxation. Many countries have mutual tax agreements, so that double taxation won't occur or be minimized. Also, where there are taxes, there are always ways to "optimize" taxes. I myself might use the DTV only until I have reshuffled some income to meet the last bit of the LTR requirements. The latter comes with a tax exemption on all foreign income - that would be sweet and good to plan with.
I don't see the big fuss about the taxation. If you live in another country for more than 180 days, you are basically (roughly speaking and for most cases) switching from being a tax resident in your former country of residence to being a tax resident in Thailand. I think a moderate taxation would only be fair and to be expected, then.