Conventional wisdom is to reduce the protein % to around 15% after week 3 to avoid the risk of angel wing. You could do that by adding something, like rolled oats (for example), to the starter at a proportion that would bring overall protein % down to around 15%.
I've raised different types of ducks for a number of years, and only ever had 1 develop angel wing. He was a very fast-growing Muscovy drake and he was being fed a waterfowl feed. Taping the wing corrected the issue, but I reduced the protein in my ducklings' feed thereafter and had no further recurrences.
With experience, I've come to suspect high carb content as a more likely cause (and genetics may also be a factor) of angel wing than protein because I raised other ducklings on starter and continued to feed (21% protein) starter into adulthood, without angel wing developing.
Ducks need additional niacin in their diet, so supplementing with nutritional yeast or another niacin supplement is necessary to prevent deficiency - chick starter does not contain adequate amounts of niacin. Ducks may develop serious health problems, often noticeable in their legs, if their diet doesn't contain sufficient niacin.
While I feed nutritional yeast and probiotics, I generally reserve supplements with vitamins (other than niacin) and electrolytes for treating weak or injured birds. Electrolytes may be helpful and appropriate, for example, if birds are showing signs of heat stress in summer.
There are plenty of threads on here discussing angel wing and niacin deficiency - I encourage you to search for and review them.
Enjoy your ducklings - they grow so quickly!
