This sound like it is a flock-wide problem, not an individual chicken problem. So let's think along those lines. How are you feeding them? Brittle feathers can be caused by nutritional deficiencies, especially certain minerals like calcium, zinc, selenium, manganese, or magnesium. Individual hens sometimes don't process some of these minerals properly (usually genetic) so you can often get one or two hens with this problem, but since it seems every hen is affected look at diet. if the feathers are brittle, even a good rooster with good technique can cause excess feather loss.
It's quite possible it is at least partially the fault of the rooster. As you mentioned his spurs probably aren't big enough to cause the problem, it's usually not the spurs anyway. When they mount the hen they hold on with their claws. Their toenails can be pretty long and sharp long before spurs become an issue. It might help to blunt the end of his claws. It's a lot like trimming your toenails, if you stay out of the quick it doesn't hurt at all and doesn't bleed. The quick is different in different chickens. Usually if you take off less than 1/3 of the claw you don't hit the quick, but you might want some flour on hand to stop the bleeding just in case. I've used a Dremel tool with one of those discs used to cut metal to knock off the sharp point of the toenails. If you wrap the bird in a towel they are usually pretty calm through this process. I've heard of people using a dog toenail clipper but I have no experience with that.
If the toenails cause feather loss, sharp spurs can cut the skin if it is bare. The toenails can too. Bare skin is at risk, even with a cockerel and his toenails. it is something to be avoided.
Have you checked for mites and lice? They can cause feather loss.
If it is purely due to your cockerel's technique, it's possible that will get better as he and the pullets mature. The pullets can play a part in this too. The cockerel has to have the maturity and personality the pullets can respect instead of just using brute force to establish his dominance. Once he reaches that point the pullets have to have the maturity and personality to accept him instead of fighting him or resisting. I've had cockerels reach that point of maturity at five months, even with a flock of older hens. I've had cockerels reach 11 months before the last mature hen accepted him. it's not just age, it's maturity. Some never mature.
The only reason you need a rooster is if you want fertile eggs. Everything else is personal preference. If he is causing problems in the flock, do you really need to keep him?