All you did there was prove your assumption. You assumed spacetime, then you assumed it to be a thing-in-itself (which, by the way, is an IDEA) and then noticed it has certain properties as described by various theories (such as from Schwarzild). None of this is evidence that space or time - which, by the way, I DARE you to define - are properties of some world independent of consciousness (the witness/observer), your just merely asserting them. You can say they are empirical (or phenomenological) facts, but that is much different than saying they are things-in-themselves