I went on a long search for anything that had some similarity to the series. To be honest, it's hard since "The Expanse" series is an interesting combination of a few different things that make it unique.

But, I did eventually find a few other series that had their own unique qualities that managed to leave me with a similar feeling when completed. Most recently, the Dead Empire's Fall Trilogy from Walter Jon Williams (and the second trilogy in the same universe). But, I was also really impressed with John Scalzi's Interdependency Trilogy as well. And lastly, the Murderbot Diaries from Marth Wells was a whole bucket of interesting twist on a scifi world that left a truly unique impression on me. Though, the last example is still not complete with a new book coming out this year or next year I think.

I'm sure the Germans have some enormous compound word for the loss you feel when you finish a great series of books or tv.