I address that kind of inner mysticism in an upcoming article on Schopenhauer's pantheism. But I'm not sure you're disagreeing with what I say here about "nothing."

Instead of saying, "Remove every particular thing to arrive at nothing," I say that when we recognize the subjectivity of the simplifying conceptions that define all particular things, we're left with no mere thing, as in "nothing." That nothing is Kant's noumenon, the thing in-itself, which I treat as the cosmic whole.

The problem with mysticism, though, is with that leap from inner to outer reality. Just because a certain mental state seems paramount to us or to spiritually disciplined individuals, doesn't mean the rest of the universe must conform to it or that that state is the source of all things. That's still a kind of anthropocentrism that conflicts with the cosmicist (Lovecraftian, existentially alienating) upshot of science, as far as I can tell.