In my experience, PhD students tend to be far more anxious and stressed-out than people in regular jobs, which used to surprise me, because you generally have more autonomy and flexibility in your schedule than in a regular job, you're much less likely to get fired than in a regular job, and often you work lighter hours than in a regular job (although this can vary hugely by field--in my experience fields like math and literature where you pretty much sit in a room and read and do your own thing are the ones where hours are shorter, whereas in fields like chemistry and biochemistry where there's a lot of concern about findings getting "scooped" and the work requires a lot of very time-intensive experiments, the hours can be extremely long).
But in about year 4 or 5 of my PhD program, I finally generated a grand holistic theory about why grad students are so damn stressed out: it's because most of them desperately want to be professors, most of them won't be able to get jobs as professors (because there are vastly more grad students who want those jobs than jobs available), and whether or not they actually get one of those jobs is largely due to factors outside of their control, a major one of which is the opinion of them held by one person--their PhD adviser. So it's a very interesting culture--unlike regular jobs, where people won't say FU to their bosses because they don't want to get fired and they desperately need the money, in grad school people won't say FU to their bosses because they desperately need their boss to think highly of them so they have a hope of getting a job as a professor.
That means that if your post-FI career ambitions don't involve getting a job in academia, you have more latitude to say FU than most students. But if you do desperately want the PhD for bragging rights/for your NASA plans, you still can't totally say FU because your boss still determines whether or not you can get that PhD, and you still have to keep working with said boss for the five-ish years it'll take you to get that degree.
Also, I know that you mentioned that one of the reasons that you want to get a PhD is because you want more freedom to pursue your own research interests. I don't know engineering specifically, but in any field that's largely lab-based (ie, any field where you do experiments where you have to get grant money to pay for those experiments), your research is going to be very restricted by the kind of research that your adviser is interested in/has funding to do. Of course, if you know exactly the kind of research you want to do, you can find an adviser who's doing that kind of research and make it happen that way. If you're a professor at a research university, then you can do any kind of research that you can convince a grant committee to give you money for, but, again, it's often a hard and anxiety-ridden process to get those jobs. In the paper-and-pencil fields, my understanding is that PhD students get a lot more latitude in choosing their research projects. And in the lab-based fields, my experience is that very senior, well-established and well-funded professors tend to offer their students more research freedom than more junior professors who might only have grants for one or two small projects and are really working to establish themselves as a name in a particular area.
So, my biggest piece of advice is, if you're going to do it, pick your adviser very, very, very carefully. Find someone who a) is doing the kind of research that you want to be doing, b) is willing to give you a fair amount of freedom, and c) won't make your life miserable. A lot of what FI is about for many people is that great feeling of knowing that you're not beholden to a boss. When you enter a PhD program, as long as actually getting that credential matters to you, you will be beholden to a boss again, because she'll determine whether or not you can get it. So the best thing you can do is find a boss that will make you feel as comfortable with that situation as possible (that is, one who you think will let you do what's as close as possible to what you actually want to be doing).