I see you updated your original post to include the Plaguebearers, I made a post a while back suggesting them and detailing their role if you would like to take a look.
They would be great alternatives to Crushers with a nice Nurgle theme, I think they should be more common than just boss encounters though, as they are the frontline lesser demons for Nurgle.
*edit: fixed link
Appreciate the kind words and featuring the post link