For what periods of time have you been able to sustain your needed income? I did this for a few years, but found that it often took a significant portion of time to line up the gigs, and some customers would hold payment for extra work.
Do you ever end up with only 5h of work when you want / need more?
The economy was diff a few years ago, though..
I had some relatively steady long-term clients. Sometimes they were slow to pay, but they always paid eventually.
I had enough cushion from the previous job not to worry too much about short term cashflow. Otherwise it can be a problem.
I've gone for periods at as little as 5 hours a week. In that case I was mentoring more junior programmers, rather than owning a software project myself. It worked out great and the client was happy.
As you saw, at 16 hours you can easily own a software project and get great results. There's no reason you can't bid for freelance work at that level of commitment.