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.

We can treat human responses to cognitions as involving law-like connections grounded on free choices which show themselves in our character.

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.

People who enjoy the privileges of success must use these privileges to benefit those who do not have them. These privileges constitute a deep hole they need to climb out of if they are to prevent its being the case that the world would have been better off if they had never been born.

Fichte is a necessary step to both Hegel and Marx.

When people think that moral problems can be solved by some simple strategy of calculation, that sets them up for ghastly overreaching. They think they can turn everything into a "science" the way mechanics was turned into a science in the seventeeth century. They want to turn everything over to technocrats and social engineers. They become shortsighted or simplistic about their ends, and they disastrously overestimate their ability to acquire the information they need to make the needed calculations.

There is a tradition of "modernist" theology arising out of post-Kantian thought - Fichte was the real father of it, but Schleiermacher and others also developed it - which might have more promise if it had greater influence on popular religion.