Hi and welcome.
Thin shelled eggs can be linked to a hen coming to the end of their laying life or bad diet. Some hens also seem prone to it, but I would have thought caged birds would be dispatched fairly quickly if they kept laying them.
Hybrid layers are bred to produce eggs daily over a short life span, so the ex-batts you have will have much shorter life expectancy and laying life than your bantams. Growing a whole new set of feathers also takes it out of a hen and could account for a lot.
They will have also been fed on layers pellets, so any change to their diet could upset their digestive systems. It doesn't look like worms in their poop, but it could be undigested corn. If you are feeding this, I would stop for a while. All new foods need to be introduced gradually and they need access to grit so they can digest corn. Layers pellets contain calcium that should help with the thin shells, but you could also offer oyster shell grit (as well as normal grit).