Methodology of defining a research topic
I think this is a great exercise.  One of the hardest things to cope with in the transition from graduate student to PI is broadening your focus as you move from a "worker" role toward a "manager" role in research.  This doesn't mean you're not going to be continuing to research, but you'll likely have a lot less time to do it than you think, and you need to prepare for how you will supervise where previously you would execute.

That said, I think you're currently approaching this too much like you are going to be the one doing the research, rather than their supervisor.  I would suggest limiting yourself to finding a topic, milestones, and rough criteria for success: the rest is what you want to coach a student as they look into the background, compare in depth with alternate approaches, etc.

The other thing I would advise is that you need to separate your research goals from the goals you want the student to work on.  Every student is a gamble: some work out well, while others drop out, burn out, should never have been in the program to begin with, or just plain aren't a good match for you or your research area.  For this reason, I believe you should never have a student be critical path on an aspect of your research that deeply matters to you: that will put you in a position where you are likely to be trying to push them and micromanage them, because you are worried about the success of the work, rather than the success of the student.

Instead, think about the "paths not taken" on your research, the things that you think would be interesting and scientifically valuable, but that you aren't going to do, the sort of stuff that pads out many papers' "discussion" and "future work" sections.  These sorts of "sibling projects" to your critical path are likely to be a rich vein of seeds for student projects.