Baking eggshells first is a step I never thought to do. I just gave them raw shells for years and never had a problem. The protein fix is the biggest deal though, my birds looked awful until I switched to a higher protein layer feed. A handful of mealworms as a treat seems to keep them busy and off each other's feathers too.
Yeah, the protein thing is huge for feather regrowth. Adding some flaxseed to their feed for omega 3s can make feathers super shiny and strong too. I also sprinkle in a poultry vitamin with biotin sometimes, it's like a multivitamin for their feathers. Good gut health helps with absorption, so a bit of plain yogurt or probiotics now and then makes all the other stuff work better.
Mix a little apple cider vinegar in their water to help with gut health. It makes the good stuff in their feed easier to absorb. @hunt.rowan is right about probiotics, but I find apple cider vinegar is cheaper and does the job. I also give crushed eggshells for calcium, which really helps feathers stay strong. Just bake the shells first to keep things safe.