Maybe you would find it less unsettling - and I believe you would be better understanding Kant's ideas - if, instead of thinking about it as "an underlying real

Maybe you would find it less unsettling - and I believe you would be better understanding Kant's ideas - if, instead of thinking about it as "an underlying reality ... which we can never know", you think of it as "something (or nothing!) underlying reality ... which we can never know". So, yes, Einstein and Newton made statements about reality. And, yes, all of reality is (theoretically) knowable to humans (including the nature of time). But maybe there's something beyond reality itself? (Or maybe not!) Newton proved that we can understand gravity. Einstein proved that we can understand electrodynamics. Kant proved that we cannot understand god(s) (including whether or not god(s) exist(s)), because god(s) is by definition beyond reality, beyond space / time, beyond the observable universe.