I'd cut out all but the layer crumble(what brand/model?) and lots of fresh plain water.They have access to their layers crumble (wont eat pellets?) Which has the right amount of protein, corn, wheat, meal worms, and a small amount of suet treats (advised by vet). On cold days they get porridge mixed with wheat. They just dont ever put weight on?
Maybe the occasional mealworm treat.
I assume you have a way to keep the water thawed at all times?
My vet isnt too worried as they cant find a cause but would be nice for them to bulk out a bit.