I'm not sure where you're located, but if you're somewhere that's warm right now, let them out to graze

Geese evolved to eat grass, and if you can let them out to graze, they will eat it up. They actually prefer grass to feed - this time of year, I let my geese out in grazing pens, and they barely touch the feed I leave out for them all the time.
Additionally, I would cut out the corn. Corn has very little nutritional value.
20% is also not a dangerous amount of protein. I usually grow my geese out on a feed that's actually 22% protein, and have never had issues with angel wing. Sometimes there is a genetic component to angel wing, and males are far more likely to get it than females, so it's possible that the three you have that did get it were genetically predisposed and also males, making it more likely to have happened.
All that said, what do you have for feed stores near you? Any
Tractor Supply stores? If yes, you can purchase NatureWise chick starter there, which, even though it's for chicks, has plenty of niacin, more than 3 times the base recommended amount for ducklings, in fact, and also is a little lower in protein at 18%. You could get this and feed it to them.