A Nellie Stevens would never work in that position. It is way too huge. I agree with others that the solution is a trellis or mechanical shades of some kind. A trellis could be as primitive as cords on hooks in the porch beam to something made of wood or metal installed in the ground. Heavenly Blue morning glories would be a quick climber. Though not permanent they could get you started.

You have to match plants to the conditions you're offering. Do drift roses love dense shade? I don't think so. So under the tree would be a bad place for them. Put them in the sunny areas. Put shade tolerant groundcover below the trees.

By saying, "... and am completely overwhelmed trying to decide on plants ..." makes it sound as if you are wanting to "garden" the slope with a variety of plants. I wouldn't know what plants would work for you in that way, My suggestions are conceptual and about what minimum I think you ought to do. First, I would add one tree to the slope in order to make the tree scheme consistent. It looks like one is missing, Closer to the house (I presume that this is a yard and there is a house behind the camera), alongside the wall, might be a spot for an additional, smaller tree. Also, I'd bury the rocks such that their top surface matches, or is consistently slightly above the elevation of the lawn edge mowing strip. Third, plant something to cover the ground. Whether it's a uniform groundcover or a variety of plants would be up to you. However, I wouldn't place plants with any height to speak of at the foreground of the bed. Those would be better off closer to the perimeter wall, as part of the backdrop.

This is the kind of project where paying a garden coach, consultant or designer to come to your home in person and help you figure out what you want to do next would be optimal. Being on the planting site themselves would also enable them to view the current effects of the flooding soil situation and decide what needs to be done in response.