Meat birds have loose stools. It's a common problem. Some people claim relief by fermenting the feed, but I don't think that is the way to go when your goal is to put on weight. Yes, 8 weeks is pretty much tops with Cornish cross. We did 12 weeks with red Rangers and probably used twice as much food as Cornish cross does.
I would trust your butcher to immediately see if there is a problem, more than likely I don't think yours are sick.
If you want them to go out, you just have to build them something where they don't have a choice and they are always out. We put ours in a tractor from 3 weeks old (our weather was good at the time.) they do not eat grass though. So if you want to cut the feed bill make sure the forage is clover and other protein rich "weeds" basically.
If you do give it a go again, the typical pounds of feed conversion to pounds of bird is Cornish cross (or cornish rock X) 2:1
Red or freedom Rangers, pioneer, other cross breeds 3-4:1
Heritage breeds 4+:1
They will eat table scraps, but only protein converts.
we had a rough last week before butcher because our 22 roosters were fighting and finding their voices at 3am....there are always things you learn to do differently for next time