It is actually a nice question how far Descartes himself endorses the monological and metaphysically dualistic theory of mind associated with his name and his legacy in early modern philosophy. But Fichte does reject this tradition, by suggesting that an immaterial thinking substance is an incoherent notion, and a rational being whose rationality was not developed through communication with others is a transcendental impossibility.

If the problem of free will is to see how freedom fits into the order of nature, then Kant's basic view about the free will problem is that it is insoluble.

One rational standard of action is how well it promotes the end it seeks. Another standard is whether it aims at ends which are good. Both of these, but especially the former, depend on judgments of fact.

Fichte is concerned with freedom as non-domination.

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.

Kant did think he had a moral route back to rational faith in God, for those who need it, and he thought that at some level, we all do need something like it.

There is no author or legislator of the moral law. It is simply valid in itself in the nature or essence of things. We become autonomous only when we obey it, because then our will aligns itself with the objectively valid law, and our choice follows the same law as that we give ourselves. We can think of rational faculty (or the idea - the pure rational concept, not exhibitable in experience) as the legislator or author of the law because reason recognizes an objective standard, and to that extent is already aligned with objective moral truth.