Do your dogs dig under the current fence???
You have 900 feet to protect from digging predators. Most digging predators dig, because they are not able to go over the top of fence. That would be a secured small run that has top covered/secured as well.
No matter what you try with the perimeter underground, your 900 foot fenced area is still not predator proof for the obvious overhead area open. I suggest you have your coop inside a smaller well secured run that has the top covered as well. This would minimize predators entering.
Most predators that is. Some are nearly impossible to keep out, like mink, and weasels and such from that category . Maybe you wont have those predators to deal with.
Free range in the larger area under some supervision, or when you know the safe cycles. (when you are pretty sure predators will not be there)
I would skip securing the 900 feet for now, and concentrate in the smaller secure run. There are also much less expensive options to secure long perimeters. (Hot Wire works well)
Only effective protection from hawks, is overhead wire/netting.
Nothing is 100% in life. Best we can do is TRY OUR BEST.

Ask anything else you are not sure about....
and WELCOME TO BYC,,,,,,,,,,,,