Yeah, you don't want pumpkin. That's fiber/bulking. On the other hand, you might want to try.. pumpkin-like soup. few spoonfuls of pumpkin.. double the olive oil and lots of water. If it's watery, it might be enough water to stay in the body and keep the stools soft. But being on a pumpkin-oliveoil soup diet isn't exactly healthy.
Canned food might also work. My dog is raw fed but canned food just makes nasty, mushy stools which is why people hate feeding it. LOL Again, add olive oil and water to it.
Oyxpowder works for dogs the same way it does for people. It softens or liquefies it while it's IN the colon. What texture you get depends on how much you give and the dog drinks.
I went through something of this nature with my dog. He tore the lining in his anus from passing a really hard/big stool. Passing stools caused him so much pain he was holding his stools in. He'd squat to go but wouldn't push it out. He got bloated, lazy, fatigued, panted a lot.. etc. Classic constipated/feel like crap symptoms. He's 24lbs and I had to give him 4 capsules which oddly is how much I take myself! Less didn't work at first.
A week or so on Oxypowder keeping him soft and regular and he got his energy back and passed soft, easy to pass stools.
You can't really ask a vet about it since they won't know what Oxypowder is. I used it because I know how it effects me and trusted it. A vet would probably just prescribe a stool softener or laxative for the same effect.