Wow... I have some suggestions but the answer seems to be that you will not want to let them free range until they are nearly adults.
My situation is similar in that I have had mother hens hatch ~100 chicks over the last few years. Each time, I let the mom take them out when she's ready, which is usually just days after they're born. I free range too, secure coop at night but no fence during the day and on the edge of 3,000 acres of forest which is filled with your normal predators (fox, coyote, hawk, etc). But the moms take them out and very quickly, up into the woods and into the pastures. Usually, she sticks close to nearby cover but over time, they venture further and further. I have 5 dogs too but all have been taught to co-exist peacefully withing feet of the chickens.
So... for what it's worth and in case it helps, it might be time to teach the collie to not herd/snap at any chickens? Help her understand this is not appropriate behavior?
For the hawk danger, do you have any roosters with your girls? Between the dog and the rooster, they can do a pretty good job of just being a deterrent from hawks. I have had over the years, 4 juveniles taken by a hawk in one period. So, it's the 5 dogs and my roosters, I think combined that keep the babies safe from hawks.
For the feral cats... this is probably the most difficult to address. I don't have any experience with this as we don't have this issue....
In any case, hope this is of some value to you,
Guppy