I’m sorry but your numbers don’t add up. You had 6 new ones, three are dead, one was removed to heal, and you have 5 left?
There are different reasons for this type of behavior but the usual cause is overcrowding. How many chickens do you have and how much room.
How old are the new chickens? Some of us have no major problems integrating fairly young chicks with the flock but sometimes that ends in disaster, like your attempt. Sufficient room again is often a big part of the problem. Sometimes though you just have a hen that is a true brute to others, especially new chicks. It’s often a hen lower in the pecking order, trying to not be knocked down any further though it can be the dominant hen too. With living animals it can vary.
When you add new chickens you can shake up the pecking order. I suspect the original ones are picking on each other because the pecking order has been shaken up and they need to sort that out again. You may wind up with a different hen as dominant hen.
Sometimes pecking and feather picking can be caused by poor nutrition, usually reduced protein. Crack and vegetables are usually pretty low in protein, depending on what is in the crack and which veggies you are feeding them. I really don’t think this is the problem but you might try feeding them less treats and rely more on the main chicken feed or better yet, feed them a high protein treat like tuna.
Bright lights can cause this kind of problem. In a run there is not much you can do if the sun is shining but if they spend much time in the coop you might think about that. This problem happens more in the brooder though than coop and run.
Sometimes there is one instigator that starts the attack, then mob mentality causes the others to join in the attack. If you can determine that one specific chicken is starting it, you can sometimes stop that behavior by removing that chicken. If you can isolate her for several days she loses her place in the pecking order and has to regain it when she is reintroduced. She may never regain her former status but will be too busy just working on the pecking order to attack the younger ones.
I could easily be wrong but it sounds like you are trying to integrate immature chickens with mature chickens in too small a space to start the problem, then the pecking order with the original ones was upset.