I've always been for beyond the Cosmere, beyond Adonalsium theories, so I think you've made a solid argument for a finite Cognitive Realm but not much of one for a round Cognitive Realm. Let's see if I can help with that.
The Cognitive Realm is shaped by the collective consciousness of the inhabitants of the Cosmere. It is composed of and thus bounded by Investiture, presumably. We have no indication if other Adonalsium-lite beings might be out there, other... energies equivalent to Investiture, or if the space beyond the Cosmere is completely Investiture null. We and inhabitants of Cosmere don't know what things are like beyond the Cosmere. Maybe the nearby galaxies are the cytoverse and Reckonersverse, who knows? But how would one visualize something that they can't reach? On Earth, a common analogy for that across many different cultures was the heavens and the stars. So the Cosmere is surrounded by a multitude of galaxies, other worlds... and they are completely unreachable. They are bounded, on every side, there are worlds out there, but beyond their reach, how might they visualize this? A sky full of stars that they are looking up from their world, the entire Cosmere, surrounded by the universe, and thus their own little corner of the universe is visualized as a sphere.