No insurance needed for a non-O retirement visa, but to answer your question, no, Medicare cannot be used to satisfy the insurance requirement for a non-OA retirement visa. There are only specific health providers the government requires you to have in order to satisfy the requirement. As for self insuring, just weigh the benefit of paying a small amount for outpatient services as you need them vs a large health insurance premium that might save you financially if you are ever in a catastrophic accident or have a stroke leaving you in the hospital for months. Up to you.
You’re better off with a retirement visa for coming and going the whole year. Immigration will frown on you coming back every other month on a visa exempt or a tourist visa.
Unfortunately, at your age you’ll be lucky to get insured, and if you do, it’s going to cost you. The time to get it is before you turn 65 for it to be affordable.