You might as well read Dune Messiah as it's such a tiny book compared to the first one.
And then...
Honestly I love Frank's books in my view they get more interesting as they go on (I would have loved to see Lynch try and make the God Emperor work) but nothing will compel me to read any of the non-Frank books.
And Darth Ender if you want to understand politics (or indeed much of what I post) reading Herbert Snr and Robert Anton Wilson are good primers.