There is a big difference between physically being in the US and simply returning the proof of life form that Social Security will send to your Thai address.
You do not have to spend any time in the US. I’ve been on Social Security in Thailand since 2018. I was in Thailand four years during Covid and never in the US more than 2 weeks per year when visiting. I think other countries have rules such as you described but not the US.
I only have the free Medicare Part A. I declined to sign up for Medicare Part B. You can’t use Medicare here and I am committed to staying in Thailand. I would live here a year or two before completely cutting ties with your home country. I even continued to pay California state taxes the first couple of years. If you want to pay for private health insurance then do your shopping and buy in as early as possible. Your policy will exclude preexisting conditions so have savings and/or income to cover those expenses.
You want to apply for a nonimmigrant O visa. After 90 days you can extend 1 year at a time. When you extend you can choose retirement or marriage (assuming you are actually legally married). There is more documentation required to extend as marriage and it may require a home visit by immigration to check to see that you are really married and living together. Every immigration office is a little different so you have to check the requirements for your particular office. To extend as retired is more money but you can complete a 1 year extension same day in a couple of hours and it requires less paperwork.
800,000 2 months before you apply, 3 months after you apply, and the balance can’t go below 400,000 for the rest of the year. You then bump it back up to 800,000 2 months before the next 1 year extension. I just left the balance at 800,000 all year long. The money is yours. If you decide to leave Thailand then you can withdraw it.
First I followed questions and answers on the “Thai visa advice” Facebook group. Second, just personal experience over 7 years. YouTube videos are often wrong or incomplete. The ASEAN now web chat site allows too many conflicting opinions and is confusing. You also have to be careful about visa questions that don’t give enough detail to generate accurate answers.