I've never had problems with breaking, my shells have always been almost too strong, hard to crack the shells for frying eggs without goofing up and breaking my yolks. I've always fed lots of alfalfa, plus the egg shells back, plus of course oyster shell on the side. Seems everywhere I've lived has had a really high amount of limestone in the ground and water as well. I've had chickens for over twenty years in CO, OK and here in AZ.
Recently, I started adding clippings of rosemary, lavender and lemongrass for nesting boxes. Makes really nice nest material. I dont know why i thought of it but i do know that the herbs are supposed to deter parasitites like mites and lice. I also found it makes the coop smell really,really nice! Plus, I have huge amounts because they all three grow like weeds in my garden and need good prunings.