Agreed. I've read about half the books on this list and wouldn't consider them difficult. But I've got some new books to buy now, which is always a good thing.

If you like Ubik, Martian Time Slip might also pass as difficult.

PKD's VALIS, Transmigration, and Stigmata are the ones people usually propose for the postmodern canon. I honestly think stuff like this will probably age better than say, DFW's Infinite Jest. The Exegesis[1] is less known and difficult in a different way since he really lost his mind and isn't even aiming for a narrative.. pretty just much unedited journal entries.

https://en.wikipedia.org/wiki/The_Exegesis_of_Philip_K._Dick