If it helps reassure you, our vets (very piggy savvy) said that they recommend a hay and lettuce only diet to bladder pigs. It’s balanced without being too low or high in calcium. Small amounts of grass or other low calcium veggies can be given depending on the individual.
Perry is an indoor pig. He has meadow and timothy hays (vets recommend meadow hay from a bale and timothy hay alongside), lettuce twice a day with small amounts of cucumber, pepper (mostly green as the lowest sugar option) and every now and again a small amount of grass.
I don’t think the vitamin D is a problem to be honest. Hays that are more yellow/golden contain more vitamin D than the greener hays (more vitamin C) but I know you can’t know for sure how much.
The vitamins and minerals in pellets deteriorate very quickly.
I don’t know if that’s any help but just what I’ve been told for my own bladder piggy