Chelsea I don't know if yours forage for a lot of their food or if they are totally dependent on you for everything they eat and drink. Plenty of people worldwide have chickens that forage for everything they eat during the better weather months and their chickens do fine. In winter weather they supplement what the chickens can find foraging. That will probably include kitchen scraps or grain they raise themselves. Any grain, not just corn.
It sounds like you are not feeding them primarily a commercially prepared chicken feed with specific amounts of protein, fiber, calcium, fats, and everything else perfectly balanced for their health. Instead you are feeding them what you can. They can do quite well on that kind of diet. You will probably not be able to raise chickens that will win a grand prize at a chicken show as those generally require a specialized diet, but your chickens can lay eggs, raise baby chicks, and do quite well. What you want to try to do is to feed them as many different things as you can so they can get different nutrients from different things. I'd consider bread and yogurt as good things to help vary that mix.
When they can forage they eat a lot of green stuff whenever it is available every day. They also eat fruits, berries, and vegetables if they can get them. I consider your access to expired veggies a great benefit. That helps vary what they eat. It helps them get a balanced diet.
Someone somewhere can always come up with a reason to not feed a certain food. I don't care what the food is somebody can find a reason not to feed it. There can be a basis for some of those. I avoid green potatoes that have turned green because of exposure to sunlight, those do contain a harmful substance. But potatoes that are not green are perfectly fine, quite nutritious, yet some people say don't feed them. Even the most nutritious of treats we feed them can contain a substance that will harm them if in large enough quantities. Cabbage is a good example. It contains a substance that can harm the thyroid. But their crop cannot hold enough cabbage for it to be a problem. You would have to feed nothing except cabbage for days or probably weeks straight for them to eat enough to harm themselves.
Where I am headed with all this is that I think you are doing fine. I'd keep doing what you are doing, just try to vary which veggies as best you can.
