I find too much wheat makes them pile on fat under the skin, so I mix it with some barley, and pellets, either growers or finishers, they also get kibbled maize, but mostly they are free range and anything they find is on the menu, plenty of grass.
I also like my birds to have a stress free end, I don't do anything fancy, just go in and pick them as I am ready for them, I try not to do too many in one day or it becomes a chore