I think the term "Kantian constructivism" as an oxymoron. Kant was a constructivist about mathematics, but not about ethics.

It has taken me a long time to realize where I most disagree with Marx. His assessment of capitalism is far too favorable. He took its instability, inhumanity and irrationality to be signs that it was a merely transitional form, which had delivered into humanity's hands the means to a much better way of life than any that have ever existed on earth. Marx could not bring himself to believe that our species is so benighted, irrational and slavish that it would put up with such a monstrous way of life.

Kant thinks of judgment as a special faculty or talent of the mind, not reducible to discursive reasoning but cultivated through experience and practice.

My own view is that Kant's conception of the duality of the good (morality and happiness, the good of our person and the good of our state or condition) is a distinctively modern view.

The moral law is simply the way we think our own freedom as self-determination.

It is rational to choose the right means to your ends to develop very elegant abstract formal theories of rational choice, and then turn these into what look like moral theories. Philosophers tend to be ravished by the formal beauty of such theories, and they don't pay much attention to the fact that our human limitations make them pretty useless in practice, while the simple point about instrumental reasoning is too shallow to be of much real moral interest.

Consequentialist theories pretend that we can set some great big ends (the general happiness, human flourishing), provide ourselves with definite enough conceptions of them to make them the objects of instrumental reasoning, and then obtain enough reliable information about what actions will best promote them that we could regulate our conduct by these considerations alone.