I haven’t done Cornish cross so this is just a thought. I am told that broiler feed not only has more protein, but also Niacin which their fast growing skeletons need or they can get leg issues. If you do the low protein stuff I would add the extra Niacin for strong legs, just as you would for ducks.
The key, in my experience, is using a MHP and ditching the heat lamp all together. I like the cobb 500 strain, they stay active longer, till about 5-6 weeks. if they get into a normal sleep cycle, they will self restrict to eating only during the day and can be left with feed 24/7 without leg problems. the MHP vastly simplifies and balances the whole thing. I transition to grower formula by day 10.