Joyce's work changes over the course of his life, so I don't know what in particular you enjoy (other than, of course, that he is a fantastic author) about him, but Ulysses is a huge step from his previous work. Not to discourage you, at all, but it is helpful to review what you have just read after finishing a section by seeking out annotations, as a lot of the subtleties are easily passed over. But not before you read it for yourself.

If you like his work, though, you're probably gonna dig his contemporaries, too.

Swann's Way is the first of seven volumes in Marcel Proust's book In Remembrance of Things Past, which is the considered to be the mother of all modernist literature. It is incredibly good, introduces the whole "stream-of-consciousness" thing which will get beaten to death over the next 60 years, but gets caught up in the fussy bourgeoisie drama that was so hot at the time.

Virginia Woolf is the shit.

William Faulkner's The Sound and the Fury and As I Lay Dying are the American entries into the club, and are both beautiful novels as well. I would highly suggest you fuck with Faulkner before you move on to Ulysses.

You're younger than me when I first read Dubliners and Portrait, and are getting a lot more out of it, too, so what do I know. Fare the well and keep at it, dude.