HOw about a hydrangea? In your zone, you should have plenty of cultivers to pick from, so you should be able to find a bloom color that will work with the roses. Although a lot of the hydrangeas will get larger than 4', they can be pruned to keep them to size.

We have a yard that has thick oak and maple trees that cover almost the entire yard to the east there are trees about the same size (50-75 ft.) so like you we have almost total shade we also have about 50% to 70% clay soil that is hard to grow much in. The types of plants that are the best for vertical appearance are some types of cattails or similar bog type plants here I've seen cattails grow natively in soil that is nearly entirely clay that is almost always a mud and sometimes boggy. I don't know if you had thought of trying to make a water garden here we have to dig at least 3' deep due to temps below 0 degrees F but we have floating water hyacinths on a pond during the summer and goldfish in the pond that were feeder fish that have become 6"-8" long from mosquito larvae and also provide the plants with excellent fertilizer. If you have stray cats they will get the slow fish and the others seem to survive the same applies to raccoons and oppossums. The water hyacinths can take over a pond but do very well in the shade. Elephant's Ears have to be dug up but seem to do well in our shade and climate too holly has also done very well as a perinial here. Otherwise between our temperature and soil you have tried everything we have best of luck to you.

creeping jenny would look lovely hanging over any containers...a lovely light green color. If you want some color, you could try a few containers with some bright spots, like gerbera daisy, and exchange the containers 1-2 times per week with those in a sunnier locale...I do this with my porch, which gets no sun at all, because I love the gerbera daisies....

Welcome to the world of gardening! You've gotten some good ideas. Here are a few other things to consider. For color Katie ruella is a good little border plant. It's a dwarf and comes in purple, pink and white and will bloom in just about any conditions. Mexican Oregano blooms almost year round in those same conditions for me. Texas Betony is good. I see you already have Mealy blue sage that seems to be liking it there. There are other sages to consider. Also ornamental grasses such as the purple fountain grass you already have. Some grasses grow nicely under trees such as inland sea oats for me For something with a little height consider some of the Texas native understory trees. I think Possomhaw holly does very well in the Houston area. It has bright red berries in fall and winter. Mexican red bud stays small for me and blooms heavily in the spring. Anacacho Orchid Tree is a lovely little white blooming understory tree that butterflies love. You may have to make your bed a little wider to accomadate all the nice things you'll find to grow there. Sorry I don't have any experience with Virginia sweetspire. Beauty berry might be nice in the corner. It gets very large here in San Antonio in full sun, but in part sun would be smaller. Check out some of the plants on the site linked below plus a local privately owned nursery would be able to make more suggestions. Also visiting a botanical center and seeing whats growing well in similar conditions that you have is very helpful. Here is a link that might be useful: Natives of Texas