Has ANY author managed to pull off a five or six book adult fiction series with continuing plots and themes, without crashing and burning? I can't think of one offhand.
Steven Erikson's The Malazan Book of the Fallen series. 10 awesome books.
https://www.amazon.com/dp/B002KYHZLQ/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1