You cannot compare CX to any heritage. They started breeding them in the 1950s and they have kept increasing the size.
https://backyardpoultry.iamcountryside.com/chickens-101/cornish-cross-chicken/
Try resting until joints move freely, can do this after they thaw. I thought I was trying to eat rubber bands when I didn't rest a heritage bird.
If still tough , brining might help. Or marinate in something acidic to help break down the muscle fibers.... vinegar, pineapple ect