Your plan sounds pretty similar to how we keep ours. And we do take a lot of predator losses...we have decided we will have to keep twice as many birds as we really need, to account for that. Your breed selection sounds good, but if you want to keep the flock reproducing itself, you will need an incubator or a type of hen that will go broody. You might add in a few bantam hens as broodies, most are pretty good at it. That is my plan.
Keeping them locked in a secure coop/pen at night helps to minimize your losses. We have so far only really lost birds in the night and early morning hours, so as long as they stay in during those times, we don't lose any. Having a roof over their run helps a lot, that is the only place hawks get ours as it used to be open, and when the chickens are free-ranging, they have a lot of brush and trees to hide under. Predators pick out white chickens pretty easily, we find that the chickens that live longest in our flock are ones that blend in to the ground--gray or brown.
You don't 'need' to give them layer mash. We don't give it to ours at all, and they are growing, laying, and doing very well. They free range for about 8 hours a day on 7 acres of desert land (as long as someone is around to keep an eye on them, if we are not home, they don't go out, because we have every predator there is around here) and have constant access to a big tub of water and another pan that contains scratch feed, and whatever edible 'trash' we have from the kitchen. We also supplement a little with dry cat food for protein. We save all the egg shells and crunch them up to give them back to the birds mixed in their feed pan, for calcium, instead of oyster shell. I find that the birds barely touch the feed we offer them on days they go out. I fill the pan and they don't even look at it, they are so excited to go out to forage. Of course, layer feed works great for most people, and there's nothing wrong with it. If you want to go that way, by all means. Give your chickens a big pan of it in their coop and let them eat what they want of it. We just can't get it here, and to order it costs too much. Plus we are working on a project raising chickens that are self-sufficient--I want to know that if the economy or legal stuff with livestock takes a huge downturn and I can't feed them or legally own them anymore, I can turn them loose and let them feed/care for themselves but I can still live off their eggs and hunt them for meat.
When they start laying, keep them locked in where their nest boxes are for a while until they know to lay in the boxes. Then when you turn them loose, they usually will come back to the nests to lay. At least, mine do. If the gate to my hens' run/coop area gets closed and they want to lay eggs, they will actually come to my back door and make a racket outside the door until I go let them in the coop! When I had to use their coop as an emergency pen for a baby goat, the hens were so out of sorts, and they refused to lay an egg anywhere else, so they just did not lay at all until the goat was gone. They are creatures of habit, and like to return to the same place to lay unless something disturbs them there.
Low maintenance, free-ranging, self-sustaining flocks of chickens are how most of the rest of the world does chickens. Go to Asia or Africa--I bet their birds are not in spotless secure coops eating layer mash all day. They take care of themselves. "Civilized" countries have just gotten away from that in favor of more efficiency and convenience. Chickens are a great low-maintenance food provider for the family and are great at taking care of themselves if given the chance.
