Late to the party here, but we had the same issue. Rocket could only tolerate all of the richer foods for a month or two before having major issues. We tried them all over time.
The best we have found for him are Precise Foundation Chicken Meal and Rice, and Precise Senior. He just turned 8, and we were seeing occasional soft poops on Foundation after several years on it. The Senior blend seemed to do the trick.
For really sensitive tummies, their Lamb based Sensicare formula works well for some others that we know.
It can be hard to find a Precise dealer in some areas, so order it from Wag or Petflow depending who has the best price.