What is the depth of the bed from the fence to the wall? Hard to tell from the pictures.

And how solid is the wall? Dry stacked or mortared? Good foundation? Does it have something that ties it deeply into the hill? If not, I don't think it will hold up to adding the weight of trees such as arborvitae and I would plan on vines on trellises.

In a tight space like this, I would use a tree form for screening such that the exposed trunk system (more likely multi-trunk) silhouettes against the wall and the foliage does not begin until the top of the fence. That way, the "ground floor" space is not overwhelmed with shrubby bushiness. Hard to suggest what you might use though, because your pictures do not convey the extent to which you need screening. Then, planted below the screening plants, at ground level, you could use a solid mat of groundcover, of which there would be numerous options, depending on the height and arrangement you wanted.

Hang curtains from your pergola. example

Rose of Sheron and Barberry Golden Devine

A great evergreen shrub for Oregon (west of the Cascades, not sure how it does in eastern Oregon) is Baggesen's Gold Honeysuckle. It can be allowed to grow freely or can be clipped into a hedge. Mine is 6 feet tall and just as wide. We're up in the hills outside of Portland, so it's rather hardy. It is easy to start from cuttings and does well in light shade or sun.