If I could go back in time, I would...hah! So I just sat here for 5 minutes. I honestly don't know. I jumped in head-first and gave up everything at home for my PhD. Doing my PhD allowed me to live in three different countries, meet a whole load of new people, live through some intense experiences (30 hour 'days', travelling and working through 8 countries in 8 days, become an expert in subject X and give a presentation on it to world experts in 3 days etc. ;-) and grow up. I learned a lot about myself. The physics and computing and mathematics were incidental, almost. But I gave up a huge opportunity cost in going into finance or starting a company. But who knows what I would have amounted to, otherwise?
From what you're saying, none of those experiences would apply to you. You're not used to living on $15k per year, and would be working a day job along side the studies. Since you'd hold onto your regular life, you wouldn't travel for renumeration amounting to just under the cost of living and you're probably too experienced to tolerate the amount of crap a grad student gets on a daily basis.
I'm not negative about doing a PhD, I think. I'd recommend it to you heartily if you were in your early twenties and wanted to go do something unique, exciting and demanding. But from your post, you're not talking about going all-in. You're talking about doing it as a hobby, almost - a help to what you really want to do. It doesn't (or shouldn't) work like that. My advice would be to do what you really want to do, and not worry about getting an external boost or credit - which is what a qualification is about, I guess.
In fact, from what you've written, it sounds to me like you don't enjoy getting your own projects done because you are busy achieving stuff during the day. I don't think that anyone can be focused and productive 15 hours a day. Try changing jobs, taking a less intense day job to concentrate at home or finding some way to roll your own projects into your day job properly.
A word of caution: I don't know anyone who did a PhD part-time. My advice is worth what you paid for it ;-) You may get lucky and have an awesome advisor who cares about your topic. You might have some incredible idea that needs academic help to bring to fruition. If you know exactly what you want to do and can find an advisor who is as enthusiastic as you, go for it! But from your post, I guess that's not the case.
