I think you are missing the most important factor here - if you have a stroke or heart attack, their investment is not the problem -- continuing to live is. It is your life to live, but IMO if your health is tanking due to work, you gotta slow down if not stop. I'd talk to them about having them buy you out. Even if you are the founder, nobody is irreplaceable. Hand it off and get healthy. No work is worth dying over, not even your own company.