100k/year gross working only 20 hours a week is pretty badass if you ask me. The average (full-time) web developer salary according to salary.com is 70k/year.
Didn't you say you used RoR? You're okay with using other folks high-level code, but you think interacting with them will pull you towards the middle of the bell curve. Interesting.
This will almost surely be damaging to your career and education. If this means that you are too easily influenced or cannot identify a good influence, then I would focus on improving that in yourself.
Surrounding yourself with intelligent people in a field such as this is incredibly important. You are missing out otherwise.
it also means that the employer bears more of the risk if the work is harder than originally imagined.
(Now, I also agree with your points about why charging a flat rate is better; I'm just pointing out that there are also reasons why charging an hourly rate is better.)