I was trying to steelman your argument, not strawman it. If you weren't thinking of Leibnizian conceptual containment, and you want to appeal to Kant's transcendental idealism, you'll have to explain why it's so useful to organize experience by appealing to the concepts of space and time, given that space and time are only mind-dependent. Can we choose other ways of organizing experience, ones that are just as useful? Not really, and Kant would say this is because of the mind's "transcendental structure." Alas, that concept of structure will likewise assume space and time since it's not just a logical order he's positing. And anyway, logic, too, is based on empirical experience.

The Kantian idea is that some concepts are more "fundamental" than others to human thinking, and we'd have to add that they evolve in history. If you can't specify the transcendental order without appealing to space and time, that's a good indicator that those realities are mind-independent. So Kant's idealism would reduce to naturalism. That is, if we have no supernatural understanding of our mental structure, we have no reason to think we're independent of how nature seems to us. Some parts of our mind seem deeper than others because they're based on older parts of the brain and are more universal in the species that are related to us in the temporal process of life's evolution. Without some such explanation, the idealist's talk of "transcendental structure" is empty.