Have you tried Plaque Off? It looks a bit like a powdered form of seaweed and you just sprinkle a small amount on the dogs food each day, It should not upset even the most sensitive of stomachs. Have you tried Nylabone, this is a good thing for dogs with poor teeth, also steamed bones are good.
Have you tried James Wellbeloved dog food? I have shelties and have found this good for sensitive tums? Try to get a free sample to try it. I have had dogs who reacted badly to pasta so keep away from that, always have deans gastric of Johnson’s diarrhoea tablets in (even human ones are ok.