I don't think it's particularly meaningful to use "free will" for that instead of "difficult to predict." I mean, you don't say that weather has free will, even though you can't model it accurately. Applying the label only to humans seems a lot like trying to sneak in a connotation that wasn't part of the technical definition. I think that your concept captures some of the real-world uses of the term "free will" but that it doesn't capture enough of the usage to help deal with the confusion around it. In particular, your definition would mean that weather has free will, which is a phrase I wouldn't be surprised to hear in colloquial English but doesn't seem to be talking about the same thing that philosophers want to debate.