I like to suggest considering herding breeds, this is in their wheelhouse. Collies and Shelties fall into this category and are very "soft" and biddable besides making excellent family dogs and pets. Yet they are tough and sturdy enough for outdoor work. I have two Shelties and because of them my 20+ chickens can free range a good portion of every day. I have seen my older Sheltie run off three coyotes on her own - at the same time. We have coyotes in the area, but they don't come around here. She marks her territory and guards the perimeter.
The younger dog was 18 months when we got him - a rescue. We put him in the brooder pen for a couple of hours every day, adjacent to the chicken pen, gradually extending his time there. He could see, hear and smell the chickens but not contact them. Eventually he was spending most of the day out there, paying no attention to the chickens, in the same way we would integrate chicks to an older flock. When we finally opened the gates so he was sharing the same space with the chickens, it was no big deal. He had zero interest in them, they were just part of the scenery.
I don't know why this casual desensitizing wouldn't work on practically any dog. Keeping people and their reactions and emotional responses out of the picture is essential, however. Just make sure the dog actually can NOT get over, under or through the fence to the chickens and leave it alone until it realizes the chickens cannot be accessed; do this consistently for at least a month, or until the dog shows no interest in them, and all should be well.