What are the income requirements and options for obtaining a non-O visa in Thailand for retirement?

This is not much? Lol My grandma has 20kthb

Come on a tourist visa and immediately hire a proven reliable Thai visa agent. They will get you through the process under the scenario you describe.

there could be some complication with the new thai worldwide tax law, especially with remitted income. You can google search some info on that

You can get the OA visa in your home country and there is no requirement to transfer any money to Thailand (you just transfer what you need to live on). The OA visa can give you two years in Thailand with just one border bounce at the end of the first year.

What you do with the money isn't really important, but what you should be more concerned about is the fact that as an American you cannot use the income method for your initial 90 day non-O visa, or for your first 1 year extension. The ONLY way to use income for both of those is with embassy certified income, and the US embassy has not offered that for many years. That means the only way you can do it is with 800,000 baht in a Thai bank account in your name only for the initial 90 day visa the initial 1 year extension. You would need 800,000 baht in the bank for at a minimum of 5-7 months and then it could not drop below 400,000 for the rest of the year while you've been making the 65,000 baht transfers for the entire year to use as proof of income for your year 2 extension.