I know you say this is not what you're going for, but I like a porter to have a big, gritty, tongue-scraping bitterness. If I brewed this, I would bitter to 40 IBUs with Chinook! My last brown porter had 25 IBUs of Willamette and it was... lacking. Cascade will have some of that (IMO) pleasant roughness by dint of its cohumulone, but not as much as Chinook.
I agree with jsullivan on yeast. I would also mash high and short, add maltodextrin, or both; get it to finish about 1.016/4ºP. Again, it's your beer, so make what you want.