I have written on this forum about the idea of a protectorate - a state limited to the defense of life, liberty, and property. I think your concept of “constitutional anarchism” or a “completely voluntary state” is basically the same thing. You can read the previous discussions here and here.
Most anarchists argue that a state by definition uses aggression to enforce some sort of monopoly. A “voluntary state” therefore, is not a state at all, and the only way to achieve freedom is to break the monopoly on law in a given territory. Anarchists also point out that any attempt to limit the state is destined to fail as it always has in the past.
I think anarchists will usually win the theoretical argument - a society without a state would be the most free in a perfect world. But as you have pointed out, we live in a world full of coercive governments, and anarchy seems like a distant and unattainable dream. So for reasons of practicality, why not aim for “constitutional anarchism” first. In the shorter term, perhaps this is the model of the most free society possible.