I am very certain your above view [Nietzsche lambasted] is a straw man. Kant did assume and postulate but not to degrade the physical world in favor of some supernatural. There is nothing wrong is assuming and postulating as long as one qualified that transparently. To Kant, what is wrong is to take the thing-in-itself as real, and that is an illusion and if done persistently, it is delusional [e.g. God and/or soul as real].
Nietzsche criticized Kant on a range of ideas merely based on Schopenhauer’s writings and critique of Kant.
It is speculated he read Kant’s third Critique of Judgment but no mentions anywhere he read the full range of Kant’s work. As such I would not give too much credibility on the soundness N’s critique of Kant.
I think N may not even knew Kant assumed or postulated the thing-in-itself.
Here is how Kant assumed and utilized the Noumenon;
Negative employment* meant one can assume or postulate the Noumenon as some thing for some function/purpose, but one CANNOT take it as real.
This principle is applied throughout Kant’s work even when he is seemingly deliberating the thing-in-itself as if it is real.
Kant warned there is a very powerful natural inherent unavoidable impulse embedded within all humans to take the thing-in-itself as really real, thus vulnerable to being delusional.
Note, in many instances Kant factor in the idea of God and a casual [or a not serious] reader will think Kant is taking God for real without aware of Kant’s qualification ‘only negative employment.’ However, Kant is aware of his above warning not to take God [the potential illusion] as real.
As I mentioned above, there is nothing wrong is assuming and postulating as long as one qualified that transparently.
Science assumed a kind of perfection, i.e. principle of uniformity throughout the universe to ensure the related scientific theory is sound and works.
Note the concept of ‘Zero Defects’
The assumption that such an ideal is achievable and strive for it is merely to increase efficiency and profits. It does not mean one will achieve actual zero defects all the time.
So Kant’s concept and idea of the Noumenon aka Thing-in-itself is similar to what Science and others are assuming perfection and the ideals to facilitate greater improvements or understanding of reality.
It is claimed, one can get an enhanced sexual experience by fantasizing [in a way assuming] sex with a perfect woman [of perfect beauty, etc.] or man [some fantasize a perfect God].
Even N himself assumed and postulated the ideal Übermensch to facilitate and consumate his philosophy.
So there is no big issue if one assumes, postulates, fantasize as long as one know what one is doing and do not take them for real.
Note the thread of ‘Szchizotypalism’ which is the various degrees of taking the unreal for real.
I noted many of N’s philosophies are in alignment with Kant’s philosophy, but N’s philosophical ideas made up of a very partial portion of Kant’s full framework.
*Kant did discuss the thing-in-itself in another very refined perspective in a seemingly ‘positive sense’ but it is very nuanced and one has to do very detailed reflection to steer it back to its ultimate ‘negative sense’ to avoid being delusional. It is not advisable to jump into this point less one has a good grasp of a greater range of Kant’s philosophy.