Sigh I really don't have time for more gaming but I guess I need to make an exception for this one...
The books and TV series are fantastic and I want more on the same.
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.
In German and Dutch you can create any word you want. So come up with some fitting words, say "serie finale loss" and just put them together. In Dutch this would be "seriefinaleverlies" You have just created a new, completely valid word. Doesn't mean that anyone would actually use it however.
A nicer way to describe this feeling that doesn't use a compound word is post-series depression or binge-blues.