According to a website :

It is compulsory that you know how to speak and write in Thai

You must know how to recite the Thai National Anthem

But there is far more than this. You have to have permanent residence for at least 5 years (there are limited permanent resident slots). They have a points system - for example to get maximum points for education you need a PhD, to get maximum points for age you have to be between 40 and 50, etc. I just an internet search, "how to obtain Thai citizenship"

@Michelle ********

you wrote 80,000 baht - the actual amount you need in a Thai bank is 800,000 baht.

Assuming you are not going to work in Thailand, you should consider the marriage visa, O or Oa retirement visa, the Thai Elite Visa, or the Long Term Residence - Wealthy Pensioneer visa. Costs and requirements of these vary but any one of these will allow a person to stay long term in Thailand.

Did your niece have the same last name as you?

Look at Thai Elite Visa - no age requirement but you may not be able to work in Thailand with it.

@Kool ******

Yes, as a grandfather I checked for the USA requirements and Thai requirements - both required notarized documents signed by both parents.

Get the superrich app and you can check anytime you desire.

@Alex ******

For me the most important are 1099-R and 1099-SSA.

One USA person reported that he used 1099's to prove income.

@Mike *****

I thought I read if you don’t stay a whole year then you don’t have to do the annual. Correct?