I need to have yard space immediately outside my door. Not something 10 feet away and 5 feet up from the door and that does nothing to fix the flow away from the outbuilding
A wall would address the drainage, as you can change the slope behind a wall, without moving the boulders or the existing hill.
But you changed the problem from "fix drainage" to "fix drainage and give me a flat, usable lawn space." It's going to take a lot of earth moving to do that, and you might still need a wall, closer to the neighbor's property.
Good luck with the project. Hope you'll post project pics.