We are facing similar design questions. With 10 chicks growing every day, we need to figure out something quickly. We know there are a pile of predators around our semi-rural property including domestic and feral cats and dogs, and all the normal natural predators. Whatever wheels we use need to be retractable so that the coop will sit flat. I am considering a leverage system, but don't want to add to the dead loading on the wheels.
Since we are recycling other structures we don't have a lot of choice in building materials, so we can't go to undersized lumber unless I mill it all myself. I don't have that much energy.
I am considering bicycle wheels in order to move over rough terrain, but I wonder if they can handle the weight.