If you want fun politics and war in the entire solar system, I recommend Red Rising and the the subsequent 2 novels (I haven’t read beyond that). The characters are incredibly sentimental but it was really fun to read. It doesn’t start off anything like The Expanse but politics within the entire solar system and divisions between the outer planets and asteroid belt were reminiscent of some scenarios.

If you want a more philosophical and hard science exploration of Humanity expanding beyond our solar system, dealing with long term evolution and exploring sentience in other species, I like Children of Time and the followup books.

Neither are very similar to Expanse but both series are great.