The situation differs in different countries as already highlighted. I'd like to stress some further issues.
You need an advisor
Even if you don't enrol for now (which might give your prospective advisors bad vibes, my feelings are that US PhD programmes are quite formalistic), you need someone to guide, council, and, well, supervise you.
The choice on an advisor is important. It's arguably even more important as choice of the subfield.
Your advisor should be aware of your situation and be able to factor it in.
It's a risk
All the formal things (coordinates programmes, supervision agreements (Betreuungsvereinbarung, a relatively new thing in Germany), graduate schools) serve (or at least are thought to serve) for the benefit of the PhD candidate. If you have firmly defined, that you need, say, 3 papers to graduate, your supervisor cannot force you to write 5 papers without letting you graduate.
Further, some kind of a formal status at the university may mean access to hardware and labs, travel funding, supervising student projects, actual positions, etc.
I understand, why you would like to have "no strings attached" in your situation. But the situation itself is already a risk for a PhD project. You (and your supervisor) need to be aware that it might take you longer than a full-time student that is not forced to any teaching and spends 12 hours a day in the lab.
Basically, if you do your PhD part-time, you need to divide the 3-4-5 years needed for the degree by the time share you do research.
To give you a concrete example: At the beginning of my scientific career I was working 3 days a week at an unrelated programming job and was trying to do my PhD the remaining 4 days a week. I was in this mode for 2 years, during which I did around 30% of my PhD. (The "breaking in" might be a factor, you need to become proficient in the subarea you are working on. But it's definitely not only this factor.) Then, I got a thrid-party funded research position. In the next 2,5 years I did the remaining 70% of my PhD. The first phase could be even seen as some kind of a preparation for the PhD, but the main contribution was done in the second phase. Insert your numbers and estimates and do your calculations.
I stress again, that not only you, but also your advisor need to be very clearly aware of this.