I have the same problem. I like wearing pants with a slightly lower rise so the belly that spills over can hang over the waistband instead of being trapped (sorry for the image I am painting here!).
I have a pair of low-rise ponte pants with a stretchy waist that are pretty comfy. For lounging and running errands, I wear pants with a fold-over waist. I recently tried some high-spandex content pants from Athleta that were pretty comfy.
Re. jeans, I have no good suggestions. I was planning to try some premium denim made of '4-way stretch' fabric with the hope that those might offer more stretch without the sag.