Val, you garden is lovely.
Yes, my garden is shady, but I still have some sunny patches for roses and herbaceous peonies. More or less, I try to have a row of peonies, then a row of roses or clematises, depending on how sunny the spot is. Fortunately, tree peonies take quite a bit of shade. Many Clematises can take shade too
I also like Heuchera plants, especially those with burgundy leaves. They work well with deep-colored tree peonies like Shimadaijin and Rockiis with their maroon blotches. Their leaves also complement the tree peony leaves
We shouldn't forget various Hellebores; they provide interest in spring, and their leaves also blend nicely with tree peony leaves
I have several native ground covers that thrive in the shade and grow around and beneath tree peonies: native Violets, May Apples, Wood Poppies, Canadian Anemone, Solomon's Seal, False Solomon's Seal, and Wild Ginger (both native and European)
You are right; Geraniums work well with peonies, but they might have different growing habits here. For example, Ann Folkard proved to be too aggressive in my garden, especially in the sun. I like native Geranium maculatum, but it doesn't cover much space. Smaller Geranium macrorrhizum hybrids (e.g., 'Biokovo') work well beneath the tree peonies in the shade
I use regular Geranium macrorrhizum (both pink and white) in the corners where no other ground cover can grow. They can also overwinter in pots. I also have ferns in the shady corners. There are two really shady corners in my garden: one below two huge Hungarian Lilacs, and another below Sambucus nigra 'Black Lace.' This latter corner is my 'black corner,' where I have deep burgundy tree peonies, Black Dragon and Cup of Shining Night
Once I start listing plants that work well with peonies, it doesn't look too bad 😄. We have some choices. Now that I've landscaped the garden, I should take more pictures.
However, I need to include more roses in the back garden. At the moment, I have only three, all wild-looking: Rosa canina, Rosa multiflora, and Mozart (see below). They work well with herbaceous peonies. I love peonies, but a summer garden without roses...

I definitely need more roses in the back garden. I will keep you posted