What Ken said
Working as your own boss can be very rewarding. Its not for everyone, though, and as you have indicated that you are willing to work 23 hours a day, that'll do perfect
It takes connections to get the work, though. I think that product development (on your own) is very high risk, especially if you have to create demand for the product. It can take a lot of $$ to get the attention that your product deserves, even if it is a very good one.
So for a safe start, I think you need to figure out how to situate yourself in a place or market where you can get some work. Since I live in a rural area, I started out in agricultural machine repair, using manual lathes and mills. I worked at another job for a couple of years while I built up a clientel and got a good reputation. It was slim goings at first. But I enjoyed the work nonetheless: the repair field can be quite challenging and demands that you gain a lot of experience in many different areas and trade skills, all at the same time of course
You are right: straight mass production is not good entertainment, but it often pays the bills. Even in manually produced parts, making a batch allows you to get pretty good at efficiency.
I highly advocate that cnc people have manual machine shop experience first. Otherwise, you just don't have the intuitive feel for running the cnc correctly, and setting up parts.