The ramp might be your problem. My ducks never go into their coop at night until I go out and heard them in. But I just get them moving in the direction of the door and close it behind them. I can see how getting them all to climb up that ramp would be a challenge. I'd put walls around the bottom of the coop and let them make their home in there. You'll have an easier time getting them in a ground level door. Your run sounds predator proof enough that, personally, I wouldn't worry about locking them inside a physical building at night but that's just me. I don't know what kind of predators you've got in your neighborhood.
Hens go broody when you don’t want them to… and won’t go broody when you do.
but honestly more ducks will keep each other warmer. I'd put a crate in the garage.
