They are definitely still growing and will not reach their max weight until they are a couple years old.
Does the Scratch and Peck feed consist of separate grains? If so I would switch to a high protein crumble or pellet to make sure they are getting all the nutrients they need and not picking...
Good for you checking the mill dates! That has been my experience with Chewy as well. I also order Nutrena feeds from there and they are always recently milled and have never gone bad in storage. TSC on the other hand....