Look at the elite visa, or privilege visa as it’s now called, it’s probably your best option. You might be retired in the US but to get any visa based on retirement for Thailand you must be minimum 50 years old.
Australian passport holders receive a free 30 day visa exempt stamp upon arriving in Thailand, you also have the option to get a 30 day extension at immigration for 1,900 baht. So basically 30 days for free and up to 60 if you wish for 1,900 baht.
Well that all depends on your age, level of cover required, pre-existing conditions, deductible chosen, amongst other things, to get a better answer you must be more specific.