I broke the fast slowly with vegetable broth, then cooked vegetables and now rice and oats. I don't have any more ideas, so any input would be greatly appreciated on my Simba's behalf. In any German shepherd with chronic diarrhea my first instinct is to look for pancreatic insufficiency. In this case, insufficient digestive enzymes are being produced to allow proper digestion. This leads to persistent diarrhea.
Pancreatic insufficiency is reasonably common in shepherds.
There are pretty accurate tests for this condition now. It is important not to overlook other possible causes, though. Pancreatic insufficiency is NOT pancreatitis. It is an entirely different problem. Your vet may have been referring to this but phrased the concern badly or may be thinking that pancreatitis is possible.