Can I apply for a retirement visa from my home country and what are the requirements?

Like Brandon Thurkettle mentionned, if your embassy does not provide thai immigration with a letter certifying your income , for the first year you will either have to deposit 800k thb in your Thaï bank account or go through an agent. If you transfer more than 65k thb/month (every month, not 195k thb every 3 month) after the first year, you could get à letter from your bank confirming that and use it instead of 800k bht for the second year.

I’ve never heard that before

I have the 1-year non-OA visa and the application process was relatively straightforward.

For the mandatory health insurance, I recommend purchasing from a Thai agent/insurer to avoid any issues with meeting the requirements and the proof of insurance documentation.

When I arrived in Chiang Mai, the immigration officer requested proof, and I showed the complete Thai/English policy plus the receipt page with a red "PAID" stamp. Make sure to tell your insurance agent that the policy must meet non-OA visa requirements, as there are specific minimum coverage amounts that need to be met.

there couldn't be a better explanation than the one Brandon gave you. You can apply for the 90-days Non-Imm-O Retirement Visa in the E-visa system in the U.S.A., using your original income documents. Once issued the visa, enter Thailand on it, and open a Thai bank account as soon as possible. Transfer a minimum of 800,000 THB into your account. Then as soon as the money has "seasoned" for 2 months, you can apply for the yearly "12-months extension of temporary stay permit based on being over 50/retirement" on Immigration, and repeat this application every year

You're mixing up information. You can get the 90-day non-O visa from the Thai embassy showing proof of monthly income, no problem.

But once you get to Thailand, for your first extension, the ONLY thing that the immigration office will accept is embassy certified income (not available to US citizens) or poof of 800,000 in your Thai bank account.

You could get the 1-year non-OA visa if you're willing to purchase the mandatory insurance and fulfill all the other requirements (health check, background check, etc).

If your income is high enough, you could potentially get an LTR Wealthy Pensioner 10-year visa.

Or you could use an agent in Thailand to bypass the money requirement altogether.