Some archive threads here that seem to have helpful information on this topic:
https://www.backyardchickens.com/t/216685/keeping-cornish-crosses-for-layers/10
https://www.backyardchickens.com/t/263236/my-cornish-x-girls-first-birthday-pic-heavy
How large is your run? You posted somewhere else that you weren't in a position to let your birds free range right? Limiting protein will likely help, and encouraging them to forage as much as you can inside the run. I'd limit treats, and then limit them to greens and such although an...