This is why you don't retire in a country with private healthcare

3.5% historically holds up over longer retirements (I ran a rough model and got ~98% success for 50 years). $21k is quite lean - you'd have to pick and choose luxuries like a car, no roommates, travel, etc. - but for a single person in many parts of the country it's doable. The big gamble is that Medicaid might get cut further, which would definitely force you back to work.

Presumably, it would be cheaper if I had a fairly modern urban condo but my exurban property is a good $15K+ per year. Heck, I just had a kitchen fire and, even with good insurance, I'll probably be $50K out of pocket when all is said and done. I could probably have done things more on the cheap but didn't really make sense.

You are supposed to invest and keep the money working for you. Adjusted for inflation, S&P 500 returns 6.5% a year. That alone gets you above the poverty line. Recall, this is inflation adjusted so your $600,000 is growing with inflation and the poverty line income also grows over time. This does not account for any swings.