I did some digging and im not the only one with that view of ultra space. Think about it. What do we know about utlra space?
1: You NEED an ultra wormhole to enter jt. The "link cable" wormholes as seen in ORAS are a more direct "A to B", while the ultra wormholes are "A to ultra space then to whever you want if you find the right portal".
2: it links to ALL universes.
3: in the ORIGINAL sun and moon, Lusamine calls it a world parallel to ours, BUT that was when we only saw ultra deep sea, Nihilego's world. In thw ultra titles, Ultra Space is a vast network of universes.
4: your cave analogy: if each universe is a chamber in a cave system, as you suggest, you still have THE REST OF THE CAVE surrounding that one chamber and all its neighboring chambers. Thus each chamber = a universe and ultra space = the entirety of the cave system itself.
In a sense, Ultra Space is like a vast spacial void between all the universes. We see other instances of travel between universes and dimensions in pokemon, but all of them are nothing like ultra space, as they tend to be direct A to B portals, or higher or totally seprate realms altogether like Arceus' domain.
Plus, we already HAVE a realm that sits opposite of the pokemon multiverse in the form of the distortion world, where giratina holds dominion. And as we see in the games, it's not just light that came to the pokemon universes and ultra space, but Z-crystals, many of which grant powerful attacks to the pokemon that use them, and can even CHANGE ARCEUS' TYPE, not unlike his plates. Necrozma's physiology has SOME minor effect on Arceus as those crystals are literal chunks of Necrozma's body that broke off from it. It's light would need to A: light up an infinite spacial void that lies between universes and B: reach into every universe, save the ones it would later steal light from after its "fall from grace", and its z-power giving crystals would also have been scattered in such a similar manner, albiet to a potentially kesser degree as these are physical objects, not vast ammounts of light.
My argument is that Necrozma's power, reach, and importance to the pokemon multiverse. While its not on the same tier as Giratina, Palkia, or Dialga, who govern literal concepts of the multiverse, and Arceus, who made said concepts, Necrozma is still likely on a much higher range than just "universal". Again, going back to your cave analogy, its light would not only have to spread across the cave system itself, but into every single chamber. Granted, I am speaking for Ultra Necrozma, as normal necrozma took a massive blow to it's power and has to go around stealing light from universes and even solgaleo and lunala to try and recover itself. In its true state, that's what I'm arguing is higher than what is listed on vs wiki, given the energy it would have to produce to grant light to ultra space and every single universe connected to it. Even if the light pouring into each universe is only a portion of its powers that's still an potentially infinite number of universes across a vast spatial void, with some worlds even having more than one wormhole popping up in different locations, not just artificially but naturally to. And its not even just a normal light that its trio gives off, but its z-crystals as stated grant powerful attacks, Arceus can draw enough power from them to change types, and the other two light trio members' light grants visible buffs to pokemon like the totem pokemon. At the very least, I say 2-C to 2-B for its ultra form, as its only giving light and Z-power to these worlds, and not governing major concepts of creation itself, like the creation trio does.
Although that makes me wonder.....the crystals are chunks of its body yeah? But it itself is only psychic/dragon, while the crystals cover EVERY type. What is the deal with that? It doesn't change types like Arceus does, but parts of itself give of energy of every type, that can also change Arceus' type but have some variants that are EXCLUSIVE to specific pokemon who have specific moves? Pokemon biology is weird lol.