I, too, have soft shell issues. I feed organic layer with supplemental fresh vegs or fruits as available. They get to free range several hours a day. This flock has eaten oyster shell like candy. More than previous flocks. ( and it’s hard to find flaked shell). I’ve had more bumble foot in the last 2 years than in 15 years of ducks. I’ve tried changing food with no improvement in shells. Now I get maybe a soft shell in the evening from one of five hens and none from others. They always lay in the morning so it’s another thing that is off. No one I’ve contacted has suggestions that have helped. Niacin was mentioned but it’s already in their ration. They had no issues last year. My previous flock laid consistently for 6 years- until predators got them. They are Golden 300s as were my last flock.