But the point is, the "edge" we see here is completely fictional. It's a sharp border in space, marked in purple. Nothing of the sort exists in the real galaxy, so there are no rules for where it might be located.
I realize that, but the point is that, as you say, "Nothing of the sort exists in
the real galaxy," emphasis mine. Ergo, it exists outside the galaxy, and that places a lower limit on how far away it must be (in the fictional universe).