At first an OA visa requires health insurance. You will have to do this the first year. If you have then extend your OA visa for 1 year as married then, I believe, you no longer need health insurance. Check the details on this with Tod Daniels in this group or the Thai visa advice Facebook group.
If you extend an OA visa as retired then you need insurance. If you extend as married, I don’t think so.
You do have to return the proof life form. If you are using a US address then you have to know it arrived and have it forwarded to to you. I’ve never seen an option to submit the proof of life online. Some people will already be receiving SS in the US and then move to Thailand. Once someone gets settled here the it is a good idea to tell SS your new Thai address. Social Security totally supports Americans living long term overseas. Expats in Thailand are supposed to use the Social Security office in Manila, Philippines. That’s the office to which I send my Thailand address changes. Strangely though the proof of life letter must be returned to the US office using the envelope provided. I go to the expense of sending it using the Express Mail Service (EMS) which will also track the letter.
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.