Complex sentence: Rocca also claims that he and P.N.O wrote all of Rey's lyrics therefore it was easy to get rid of him in addition to maintaining the quality of their music and that since he left, in short, they have been much more successful.
Split into simpler sentences: Rocca also claims that he and P.N.O wrote all of Rey's lyrics therefore it wasn't difficult to get rid of him in addition to maintaining the quality of their music. Finally they state that since he left, in short, their music has been much more successful.