Correct me if I’m wrong, but the major concern with most owners is the risk that a high protein diet will in effect cause Angel Wing, to be more specific a high protein diet will cause a duck to grow faster, and as a result, the plumage will grow faster then the musculoskeletal structures resulting in angel wing.
My point is, ANYTHING that causes the bird to grow faster can result in Angel Wing, in the cases where it’s not genetic, the birds are often kept in small pen, overeat, and come down with it.
You can continue to feed them the 20% protein, very little studies exist on Angel Wing, but from my experience, and many others on here, you’re more likely to see Angelwing as a genetic trait, or more likely to see it with birds that aren’t getting a sufficient amount of exercise, or are overcrowded then to see it solely caused by the protein in its diet.
Cutting it with grains is just a waste of time, IMO.
"Patrick Biggs, Ph.D.
Nutritionist, Companion Animal Technical Solutions
"Raising waterfowl such as ducks and geese can be a fun addition to a backyard flock. These water-loving birds are happiest when they have a grassy area to roam and large pond or pool. Ducks and geese thrive best on a high-protein diet, such as Purina® Flock Raiser® 20%."
https://www.purinamills.com/chicken-feed/products/detail/purina-flock-raiser-crumbles
"Feed your backyard flock. Purina® Flock Raiser® Crumbles provide complete and balanced nutrition for healthy flocks and baby birds. Sustain an entire mixed flock of poultry, including starting and growing hens, roosters, ducks and geese Purina® Flock Raiser® Crumbles is also for turkeys, pheasants and quail 8 weeks of age and older."