Rachael,
You have the right idea, plant all different kinds of shrubs there. Not only would it look really awesome, but if a disease breaks out, all the shrubs won't die.
Rose of Sharon will flower for you, easy to grow. They will get the height you want.
Leave enough space, they will get wide too.
What about a fast growing holly? It would be evergreen and add interest, plant that towards the top.
There are so many shrubs you could plant there, you will get other suggestions.
I don't know how you feel about trimming, but don't plant too close to the property line if you just want them to grow natural.
For some reason, when I saw your pic the first thing I thought of was Junipers, the crawling kind that get tall.
Do you use that slope? Is it a pain to mow?
Look up spreading junipers on the internet, and see if you like any of them.
They are no maintance except for extreme drought, and look beautiful when they grow in.
Just a thought.

Rose of Sharon was a thought I had as well. There's also an abundance of viburnum you could look into. A tall physocarpus would work like 'Diablo'. Sinocalycanthus 'Hartlage Wine' is one of my favorites that blooms for such a long time. Also things like magnolia, fringe tree or some Japanese maples would work well since they don't necessarily have to be limbed up.
Sounds like a fun project!

As long as you don't have vorcious deer, you have a good number of choices. I have a mixed hedge that includes in your size range
arborvitae (look at Techny and Sunkist)
ninebark (I have Coppertina and Little Devil)
rhododendrons
Winter Red deciduous holly
Several different Hydrangea paniculata, but Quick Fire is my favorite and would provide contrast to your PeeGee
Spicebush (Lindera benzoin)
a clumping bamboo
Hardy shrub rose John Davis, but there are lots of other possibilities
Black Lace elderberry (but there are also gold-leaved ones)

Other shrubs could include:
One of the large, old-fashioned forsythia
lilacs
dogwood shrubs such as red-twigged dogwoods

In order to increase interest, I've planted shrubs with contrasting colors and textures of foliage and around the edges I've added bulbs, perennials, and clematis vines that grow up the shrubs to add another season of flowering. Mine has perhaps more evergreens than you need since I want year-round screening, but also want winter interest. Mine also is staggered so that there aren't spaces between the shrubs, and I have repeated several of the shrubs 2 or three times, though not in a regular pattern.
{{gwi:267390}}From 2013

{{gwi:267391}}From July 11, 2014 Clems etc

I have 5 or 6 rose of Sharon around my beds that are about 4/5 foot tall . I got them from my neighbor 2 yrs ago. I did buy the ninebark , Diablo. It was on clearance so I couldn't say no ,it's about 4 feet tall. I moved my willy baffin today and placed it at the top about 4 feet from my enormous lilac you see in photo. Yes I do use my upper yard, I have gardens and my children play up there . It's a step climb to the house from the street, then level. And a steep climb to the upper back yard , but then it's not so bad once your up there.. Still a slope though. I don't have a problem mowing up there. The two hills on the sides of the stairs are finally grass free :) I have lots of creeping thyme. Phlox, sedums and other creepers as a ground cover on them. One thing I didn't mention is that this are is FULL. Sun. My house faces south so my yard is sun from dawn till dusk. No shade provided until atleast 5 and it's not much to speak of...just my other neighbors oak and my privet hedge on that side. No I don't mind having to maintain things..I cut that 160 feet of privet all summer long ! But if I could find some nice free growing selections that would be great. Thanks guys , your making me rethink packing my bag and sellin the house ! I'm so tired of these guys and eyre yucky yard, but I do love my house. Keep the ideas coming:) NHBABS, that is just gorgeous!

Another view of the slope. I have had difficulties planting some trees in some areas because of the run off water. Constantly taking soil down the hill. My soil is well drained up there .. quite heavy .

Nhbab -that us gorgeous! I am hoping to have a screen similar to that one day. Our property slopes down and all of the houses, while decently far apart, have clear views into each other's homes, especially in winter. Since all of our properties slope to a creek area between us, a fence is useless. I am hoping that something like what you have created and perhaps a few well placed evergreen trees will help.

My other thought is a wall of Thuja Green Giants but I really prefer what you have. I am not sure I am that talented though. Beautifully done!