1. Based on your savings/expenses and being debt free it seems you can easily retire and not worry.
2. Turn in your resignation notice now. Just think, not having to work all day, you can spend time exercising, getting fit and in better shape...and eliminate all the stress.
3. You're never going to know unless you try. Mentally give yourself 2 years to try retirement and see how it goes. Set up a reasonable budget just so you have an idea what your expenses should look like. Of course it can deviate, but once you have the budget, you should mentally feel more confident about retiring now.
If that's you're thinking, well, you're never going to retire. Simple as that. We shouldn't even be having this discussion. As time goes on, the probability of the major illness will go up. However, you're going to have health insurance the entire time. Generally, even if insurance doesn't pay everything, you have an annual out of pocket maximum. So, for an individual, assuming you max out, you're going to be looking at maybe $10k/year (on top of your monthly premiums). Not the end of the world. If we figure your $4.5M is generating a lowly 2%/year, that's still $90k and you will have Social Security available starting in 9 years. In all likelihood, you'll be able to generate more than 2%/$90k per year with very little risk.
I say that you shouldn't delay - just write your resignation letter now and envision how great things will be when you don't have to work or put up with the silly bureaucracy, and have lots of time to get healthier and do what you want with your time.
What's the absolute worst thing that happens? You get real bored and things are not so great. You take a part-time job to keep you busy and making some income.
You shouldn't be so worried. You're in a great position.