Okay, if you permit me to paraphrase, you're characterizing your definition of "universe" as not, "everything that exists," but rather something more like "everything that's known." When you say "a god could exist outside the universe" you mean more or less, "a god could exist whose nature is entirely unknown."
The problem is, as others have pointed out, no actual religion describes a god whose nature is entirely unknown. Religious leaders base their authority and power on claiming to know!
Even you, when you describe a god who's the First Cause, are claiming to know. The god is unknown; it's outside of our understanding that you describe it as being outside the universe. But somehow you know that.
I submit that that claim ultimately lacks any meaning.
Suppose A says, "The First Cause is unknown."
Then B says, "The First Cause is a god, about whom absolutely nothing else is known."
Hasn't B just said exactly the same thing as A, in more words?