I'm just working my way through the whole Star Wars Expanded Universe (now known as Legends) collection. 270 books in total; I've only read 14 so far... So it will surely keep me entertained for a long time
As for "proper" sci-fi series, have you read The Expanse Series by James S. A. Corey? The TV show adaptation of it is great and I don't want to spoil it to myself so I've only read the first three books in the series but there are currently 9 altogether and even more if you count related novelas and short stories. Easily the best modern sci-fi for me.