I just found this thread and read it all. Interesting idea!
A few thoughts:
a) I like your idea of a feeder the bantams can access, but the Cornish Cross can not
b) You could try an elevated feeder (bantams fly MUCH better than Cornish Cross), although there's a chance the Cornish Cross could fly up and then hurt themselves coming down. So your idea of small openings might be better.
c) I would give the Cornish Cross most or all of their feed in the evening, if they are able to free range during the day. My reason: if they are hungry in the daytime, they will move around foraging, and get plenty of exercise as well as grass and other things. Then they can have their measured amount of food in the evening, and go to sleep with full crops.
d) You can look online for Broiler Breeder Management Guides. They are put out by various companies, telling how to raise the parents that produce the Cornish Cross. Although that's not exactly what you will have, you might find some of the information useful. For example:
https://en.aviagen.com/assets/Tech_Center/Ross_PS/RossPSHandBook2018.pdf
On page 93 of this one, there are photos and diagrams of what the breast shape of the males should be (they show too thin, just right, and too fat.) Because they want those birds to live and reproduce, they are deliberately stunting their growth with feed restriction. What they say for those breeders may be helpful in deciding what body condition is right for your pets.