The Expanse has given us nearly 5 seasons of what many believe to be brilliant stories based on science fiction. In the 4 seasons we’ve seen , pretty much everything that’s happened involved the Earthers, Belters, or Martians. In fact, none of the 3 factions has the capability of even leaving the solar system. There is a gateway to a different universe, but that to, so far, has had an affect on the same 3 human factions.
By many, many, accounted The Expanse is one of the best sci-fi series ever. I’v heard no complaints about the show being damaged by small universe
To me, small universe syndrome is just a talking point that some people raise when they start their list of why they don’t like a show or movie or book. If you like the stories, characters, etc, then small universe syndrome is not likely to bother you.