If you quitting would kill the company, then you have leverage. Use it. Don't threaten, but be clear that the current situation is not viable and that changes

If you quitting would kill the company, then you have leverage. Use it. Don't threaten, but be clear that the current situation is not viable and that changes are going to have to be made. Suggest the changes and see how they respond. Then depending on the response, reassess the situation (both the company's and yours). Remember - when health is at risk always put your health first.