I've learned something recently about brittle feathers as I am dealing with a hen that is having a very rough time with several conditions going on at the same time internally.
First, feather mites strike weak birds. Healthy birds can ward them off just as they can Cocci, worms, yeast infections, bacterial infections, ect. Yeast infections occur in birds that also suffer from low immune system fuctions and all of what I mentioned above effects the liver. A bird that suffers from a weakened or diseased liver has skin issues and very brittle feathers. Preening leads to breakage.
Cocci, worms, all species of mites, bacterial infections, intenal laying, poor genetics (and hatchery birds are not coming from clean blood lines) improper diet from even the best feed manufacturers, too many treats or fat in the diet, damaged intestines from yeast and other parasites making it difficult to absorb nutrients, etc., anything to weaken the bird, all of this effects the liver. When the liver doesn't function normally, featheres become weak and brittle. And you cannot tell the health of the liver without an enzyme test done at your vet. The liver can be ailing and there are no outward signs, poop or otherwise.
In this case I think adding a vitamin-probiotic supplement to the diet will help. Discontinue all grain treats and offer up greens only as treats. Add ginger and coconut oil to the diet to strength and improve feather health. These two things over time and a good molt will make a big difference and also strength the immune system keeping the intestinal tract in good health to absorb all their bodies need to thrive.
Good luck!