“In the ideal system, which Rambam says will be in place in Mashiach’s time, all these taxes will be paid voluntarily. No need for audits, fines or imprisonment.”
He makes no such statement in Hilchot Matanot Leevyanim where he writes that anyone not making the required contributions to the community charity fund get flogged and has their property confiscated by beit din. Not exactly anarcho-capitalism.