For a coop door, a small electronic actuator is honestly the safest and most reliable option. The ones I’ve is electronic actuator. They have enough force to lift vertically or slide horizontally, and they don’t jerk the door like some cheap winch systems do. You can connect them to a timer or even a simple light sensor if you want fully automated behavior. The weather resistance is good enough for outdoor use if you put a small cover over the motor. Installation is just a couple of brackets and a power line. The best part is that actuators lock in place, so predators can’t lift the door manually. For a coop, it’s practically the ideal solution.