Not a real fan of this approach. This is what's called emerging strategy where you react on what happening around you (not to be confused with agile where you look at what's happening around you and then deciding a course of action). Problem here is that you are never in control of where you are going to, and wasting a lot of energy and work switching over to the new strategy.
you could actually define the goal as a set of constraints.
Well that would be subtractive: I don't know what I want, but I don't want X & Y. You would steer yes, but it would be very broad. You're not really working towards something, you're working away from multiple things.
I find that orienting around results can help unlock whether positive or negative space (a goal or constraints set) is the better focus. In my experience, there are times when goals do not serve me, but rather hinder me. This is purely from regularly observing results. In those cases, pivoting to a focus on some well defined constraints has yielded better results. As long as the direction is the same between the two, that might still be considered proactive.