I've been through this with three different flocks.
When the egg becomes food to them, I found no way to change their minds. Mustard/hot sauce didn't work for me either.
Do a search on BYC for "Opa's roll out nestboxes". He has photos and plans for building roll out nests out of plywood. Work slick! He also has his experience posted on how much slope to make the egg roll out of sight, how wide to make the shelf to hide the egg on so they can't reach it under the curtain, what to make the curtain out of, what to put on the floor of the nest, to paint the inside of the box very dark color, etc.
I also went through the eggs cracking on being laid. Some of my hens stood up to lay the egg! Long fall. I favor the Easter Eggers for their extra tough eggs, although I like an egg basket of different colors.
Feeding them back their crushed egg shells seems to be a better source of calcium than oyster shell. I mixed the crushed egg shells in wet mash or scrambled eggs for them. It helps make thicker shells.
Good luck.