I see what you are saying in “Garnet’s Universe” about Steven maybe being too young to be thinking of such fantastical things and that he thinks he’s the center of the universe. I had a different take on it though and I’ll try to explain why without spoiling too much (I’m not sure how much of the show you’ve seen).
In regards to the fantastical story that Steven created, I think it makes a lot more sense when one remembers who he lives with. His guardians are magical beings who deal with crazy, fantastical stuff on a daily basis. They travel to ancient, magical ruins, fight monsters, and collect magical artifacts, with Steven occasionally tagging along. These are things that happen in his story too. But he only sees a small slice of the missions the Gems go on and as we lean from “The Test”, these are only the most minor of missions. So the things he’s imagining that Garnet did aren’t so crazy. Talking animals doesn’t seem too outlandish to me, especially considering that Steven has a magical pet lion who seems to be able to understand what he’s saying (see “Lion 2” and “Lion 3”). I also don’t think he thought his story was what Garnet actually did as he’s in disbelief when Garnet says that’s pretty much what happened. But I do see how that could also be interpreted as him just being surprised that his story was exactly right.
In regards to the “center of the universe” concern, I saw it a bit differently. This episode sort of represents Steven’s belief that Garnet doesn’t express her love towards Steven enough, or at least not as much as he’d like. In the story, Garnet tells her animal friends that Steven is the most important person to her. When they ask her if she’s told him that, she says no. When they inquire why, the best reason Steven can think of is, “I’m not strong enough”. Which isn’t really a reason at all. I think this is meant to represent that Steven knows Garnet loves him but doesn’t understand why she won’t outwardly express that to him. But Steven can’t just say something like that directly to Garnet, so his story does it for him. And we will see from this point going forward that Garnet does become more affectionate towards Steven. Yes, this might still be seen as a selfish thing that Steven wants, but it makes it more understandable for me. Plus, Steven just isn’t the kind of person to be so self-centered, which we will see more as he matures.
This isn’t to say that I disagree with your concerns, just that I saw things differently. Have a nice day.