What is the best visa option for an American married to a Thai, wishing to spend 6 months in Thailand and 6 months in the USA?

you just have to make sure you will be in Thailand every year in order to apply for the next "1-year extension of the stay permit" . . . . . .However, if you really plan only to stay less than 6 months in every calendar year, the best visa, that doesn't require any financial proof, is the 6-months multi entry Tourist Visa.

Application to the 90-days Non-Imm-O visa in the E-Visa System in your home country, the requirements for the two reasons (married to a Thai wife, or over 50 years old/Retirement) are as following:

*** for the 90 days Non-Imm-O Retirement Visa:

either

a minimum of 800,000 THB or equivalent in USD in any bank account anywhere, in your name

or

proof of monthly income of a minimum of 65,000 THB or equivalent in USD. For the application to the initial visa, it can be proven with original income documents of your home country

*** for the 90-days Non-Imm-O family (married to a Thai) visa:

Either

a minimum of 400,000 THB or equivalent in USD in any bank account anywhere, in your name

or

proof of monthly income of a minimum of 40,000 THB or equivalent in USD. Can be proven with original income documents

ATTENTION:

for the application to the 1-year extension of stay, for both reasons “family” and “retirement”, on Immigration inside Thailand, as a U.S. citizen you CANNOT use the income proof, at least not in the first year, because the U.S. embassy does not issue an affidavit of income.

You will need the aforementioned 400,000 THB respective the 800,000 THB in your Thai bank account as a financial proof, at least in the first year of a 1-year extension of stay permit

That is what my Thai wife and I do. Non-O visiting family in Thailand. There is no actual category that is marriage visa.

Why I can’t find such a 90 visa for American? Which category it fall in?

Retirement visa. I’m married and it’s the easiest.

I would advise the 'O' retirement Visa (if you over 50). You should adhere to the financial requirements (unless you go to an Agent) and make sure you come back to Thai just before the 'expiry' date of your current retirement visa so you can extend it again for another 12 months. Sounds like you should get the single entry retirement extension (1,900thb), and pay 1,000 for a 're-entry' permit when you go back to USA.