Hmm, well I'm not an expert on feline constipation, but I'm well acquainted with butt scooting on my carpet, so I can somewhat relate to your woes on some level.
If it were me with Peter, I'd cut out all kibbles entirely from his diet. No more dry for a constipated-prone cat! Additionally, I'd try spooning in about a teaspoon or more water mixed into his wet food each meal, and bring it up to about a tablespoon's worth at least, so he gets more water intake with his food.
Have you considered getting a water fountain for Peter? Other people have also used other strategies like placing numerous coffee cups, dishes, bowls, etc of water all throughout the house to encourage more drinking. Just don't place it near his eating or litter box areas, I've read that cats in the wild normally drink water away from food sources and elimination areas.
Here is a good site regarding your question on pumpkin vs miralax (and other remedies):
Since I've zero experience, I'm no help here, but hopefully the additional info can help you decide while someone else who does
have experience chime in.