Kant deduced most of his Critiques, but the categories were simply presented—pow! What he calls space and time are well defined but not described so we are free to describe them any way we want without violating the Critiques. We have discussed space and time often enough on FR, and obviously can’t come to an objective agreement—because they are entirely subjective.
True. This is what Einsteinian relativity tells us to expect. But at the same time, it insists that the laws of the universe are identical for all observers, regardless of their own particular "rest frames," or particular spatio-temporal coordinates.