Pieris on SW corner, full sun from 11am till dusk? Hmm....will be hard to get established and even then it will need plenty of water during the summer. Almost any conifers will be happy there, but alloted space and potential height will put some limitations on your choices. If space allow, I'd go for Chamaecyparis obtusa 'Crippsii'. It will provide nice texture and color contrast to your existing broadleaf evergreens.

Ornamental grasses with billowy plumes would give you height and movement if you don't want blooms. If you want blooms, Knock Out Roses bloom here in z7 from May until Thanksgiving. They are low maintenance, but would need more water and so that might not be possible out there in your hedge bed. Deer will eat them, however. Buddleia 'Pink Delight' and 'Royal Red' bloom profusely and have similar growing conditions. I have lavender around a few of my butterfly bushes.

Sounds just like my front yard. Actually you have the ideal conditions for many plants. Morning and afternoon sun are great and if you run a garden soaker hose under the mulch, you shouldn't have any worries about water. Everything grows in my garden with similar conditions. Astilbes make a wonderful addition and only really need to be watered diligently for the first year. Hostas will grow any where and there are a number of new ones out there now so you don't have use your Mother's (although if you are on a budget that is not a bad idea).Heuchera, you know them as coral bells, also do well but require more water if you are getting hot afternoon sun in the summer. Stay away from ferns and other woodland plants as they like moist conditions and remember that your yews are in love with acid soil so if they are thriving you should stick with plants that agree with this. You may also want to extend your border in front of the yews with a softer edge. Curved edges not only make your border more appealing to the eye, but are easier to mow around. With the extension of your border you can see if the sun comes closer to the outer edge and plant some other goodies here. Just remember to plant in clumps of odd numbers, 3 is good, and not in a straight line. You are trying to correct that look. And don't forget annuals for a colorful summer. There are so many that will grow in those conditions and you are in a great zone for many other plants that I can only dream of in New England. Go to the websites of nurseries that sell online, but don't buy from them, just get ideas. Make lists and drawings and head to your local nursery where you can get healthier plants already established for your area and the sound advice of a reliable nursery person. Good luck.

Do you have a height restriction? Or a height you are shooting for? The Tiny Towers needs full sun and will be marginal for your area and if you need much height very rapidly you will be waiting a long time for the Graham Blandy :-) The euonymus has issues.......very prone to scale and even in my very mild climate is subject to random winter die back, to the point where most retail nurseries no longer carry it. And if it lives that long, it will eventually get much wider than 24". The 'Wissel's Saguaro' is a great plant - I have one myself - but it presents a very random growth habit with these twisted arms (like the cactus for which it was named) popping out here and there. Not a smooth, narrow, columnar look at all. And would also prefer full sun. Can I ask why you dismissed the 'Sky Pencil'? It offers exactly what you are looking for. If you are patient as far as height is concerned, either it or the boxwood are very good choices.