Maybe layer some different textures -- some american hollies crosses like Foster and Savannah don't get too high and are very beautiful and not that shiny; a weeping yaupon, and arborvitae would complete the picture? The native plants would handle the soil well and keep it together I would suspect.

I can't think of groundcovers that will knit up the ground besides somewhat invasive things like liriope and pachysandra...

Sounds like a lovely place.

Rose

Wax myrtle (Morella (formerly Myrica) cerifera ) might be a good fit for medium sized evergreen trees. Red cedar (Juniperus virginiana) is also good.

Mix in a few maples (red) and their extensive root system might provide some soil reinforcements. Their roots are always a challenge in the garden! If they get too tall, cut them down and they'll resprout.