I've kept CX alive far too long. By combination of feed restriction and forced free ranging. The gate is (almost) always open to my pasture. All the rest of my birds eagerly free range. By only offering feed in the evening (to gather the birds back together, head count, obvious injury check, ensure they have the protection of the houses) I essentially forced my CX to spend the days hunting food, which helped in a lot of ways.
Still no guarantees - they are not bred for "robustness". Also, cut out the treats - they will eat enough without risking fatty liver disease from the various dried insect treats, BOSS and other high fat seed sources, etc. No scratch/cracked corn, either.
Finally, you can look to a commercial feed that's higher fiber than I normally recommend - look for 6-8%. That will reduce the nutrient diensity, making the bird eat longer for the same nutrition, which will allow the other birds more time to feed. Youcan also increase seperation between the feed and the water sources - make them walk further, they aren't fast at it - again to slow feeding.