I think that both Mill and Sidgwick are great and admirable philosophers, from whom we still have a lot to learn. I would not favor a form of Kantianism (if there is such a form) that treats Mill's or Sidgwick's moral philosophy with disrespect.

As I understand it, Kantian constructivism is partly a position in normative ethics and partly a position in metaethics. In metaethics, it is the position that ethical claims have truth values, but their truth conditions consist not in a set of objective facts to which they correspond, but instead in the outcome of some procedure of deliberation resulting in decisions about what to do.

It is absurd for anyone to think that Soviet "Marxism" is a correct application of the thought of Karl Marx. No doubt Soviet propaganda represented it this way. But who believes Soviet propaganda? It is remarkable (but maybe not so remarkable after all, when you consider their motives) that apologists for capitalism, who would not accept Soviet propaganda on any other point, are eager to agree with it on this point.

Kant's aim was to develop a religion within the boundaries of mere reason (that is, reason unaided by special empirical revelation) and then to ask about existing ecclesiastical faith (especially about Christianity, and the Lutheran Christianity of his time and place) how this revealed faith must be interpreted if it is to be reconciled with reason, and even seen as a wider (though morally optional) extension of a religion of reason.

Clearly no working class movement ever came about that was able to do what Marx was hoping for.

In any matter of moral importance, our first task, before we plunge ahead and decide what to do, is to figure out what we ought to do.

Virtues consist not only of acting in certain ways, but in ways of caring and feeling.