I think you are on the right track with your quest for more knowledge...meat birds are an investment, the way I figure. I switched mine over to whole grains at 2 wks, after their chick starter was done.
With you having a feed mill close, you are in a great position to get some good grain mixes that are fresh. I'll tell you something I've found out, though...for the past few years none of my chickens really want to clean up cracked corn. Use to be they would gobble it first thing but something has changed...I'm thinking it is the GMO and I have read studies done that indicate that animals just don't prefer it and even some wild animals will leave it and not eat it at all. Whatever the change, it makes the cheapest grain also the less preferable to the chickens...and I had forgotten that fact.
Even these meaties, who will eat anything that moves and most things that don't, will leave the corn and some of the wheat in the bottom of the feeder. When next I make a grain purchase, I'm sticking with barley as the biggest percentage and then adding some oats, then maybe will add some layer mash to fill it out.
See? I'm already doing it myself...that adjusting things when you observe they aren't the most ideal for the situation. I do it so often that I rarely even notice that I'm making those little adjustments as I go along. I think you will be the same and you will get into a flow that suites your chicken paradigm.
I wanted my CX to grow slower than is normally recommended and it is happening right on cue...but they are growing and you can see it almost daily. They are much bigger today than they were yesterday! I'm amazed at the capacity of these birds to grow and put on meat no matter what you feed them. The last group were raised on layer mash, whole grains and free range also and they were at finishing weights by 8 wks just like those raised on broiler feeds free choice. I waited for a couple of weeks to see if they would gain even more weight but they seemed to level off, plateau really, and gained no more weight.
You really can't go wrong with these meaties...I find them extremely hardy and active. I had one get it's whole flank peeled back by the rooster....I treated it with NuStock and I can't even find that particular bird anymore. Healed and you wouldn't even had known he was injured. I had a couple in the last batch that suffered similar grievous injuries and they didn't show any outward signs that they were hurt, healed completely and quickly and you couldn't tell who had been injured at butchering time.
The only thing I see on these forums that can kill these birds is overfeeding of high pro feeds and confinement to small spaces on moist bedding...that would kill any bird eventually. Take away that factor and you have a pretty easy bird to raise!