There's a lot of research that shows fermented feed and probiotics make thicker shells and higher shell weights.
Offering crushed oyster shell in a separate container for the hens to choose when the egg enters the shell gland works better than layer feed. The larger calcium particles remain in the upper digestive tract longer and make contact with the calcium absorption points in the intestine during the night when the hen is sleeping and the shell is forming.
So simply switching to a higher protein grower feed and oyster shell on the side accomplishes both.
Fish and meat scraps, tuna, mackerel, mealworms, crickets, plain yogurt, cottage cheese, BOSS are all high in protein.
Instead of mixed scratch grains or corn, switch to wheat, oats and BOSS will up the protein.
There are a lot of posts here about making heated bases. An engine block heater between some pavers works well.