My 6 are in an 8x5 house with a 10x10 run. There is a fence around the coop area, and another fence around the yard. Ducks herd really easy if you have a barrier to help you.
What about building the run next to the garden with a pop door on the run into the garden, and then the house either inside the run or attached to it. Put their swimming water in the run, so that they have to return to it for play, grain, water, ect. If they grow up there in a brooder they'll seek there comfort from there, and you can more easily put them to bed from the smaller area, it should work out well. I sit out with mine because I'm so paranoid. The run is fairly secure and the building is a steel shed so not much can get in there. But when they're loose, I don't trust these hawks coming through here this time of year, and there's a 2 very large owl, countless coons, and whatever else.
Every time I run mine out of the yard I say "Go Home Duckies" and they start marching towards the gate. They have a treat bowl waiting, which makes them pick up the pace and not argue with me.