How about a multi trunk tree with some underplantings in the foreground? Something that doesn't get too huge. I'm thinking a Himalayan birch or perhaps a saucer magnolia?

In it's natural habitat, Carpenteria californica grows in the shade of quercus douglasii and an ocassional quercus kellogii or quercus lobata. I've seen large strands of Carpenteria californica along seasonal and perennial creeks near the San Joaquin River with access to water most of the year, in the shade of everything else that grows along the creek's ravines.

What ever you decide on, pick only 3 or 4 different kind, and plant multiples of each, instead of a whole bunch of different ones. It will look better. Onesies look scattershot.

You have some wonderful plants. Is it important to have the window free from being hidden by plant growth?

Regarding what is currently planted:
White butterfly bush -- do you know the genus? Buddleias grow to 12 ft. here in Sunset zone 16. They work as a short tree.
Buddleja globosa -- grows to a 10 ft.by 10 ft. shrub.
Roses, Siberian iris and marjoram -- sun.

Of what is on hand I believe the digitalis, C. californica, Aquilegia and Ribes would be best in the dappled summer shade. They are upright growers so I would be on the lookout for a low-growing filler plant or use more marjoram along the perimeter (it looks very healthy). One thing I've noticed with natives is the smaller foliage can be monotonous. They need something large-leafed or very large rocks for contrast. IMO. Siberian iris may work if bloom is not of paramount importance.

Check mature height and width on shrubs. Nothing but headaches down the road if they are too close to the foundation. Good luck.

A Bird of Paradise plant would also look good planted there ☺️

Foxglove or tulips, but I am not sure how well those will grow in your climate. I use to stick bulbs in the refrigerator for one month then replant them every winter. Can violets grow where you are? Buy some seeds, soak in water, then stratify them in the refrigerator, then plant and keep the soil moist.

Here's an exotic: Malay Apple, it has beautiful flowers, and will likely do better with some shade in Southern California (because of the lower humidity).

That sounds nice, I think the Coyote mint would be nice mixed with marjoram then put carpet flower in front. The fuchsia flowering gooseberry will look nice by the bush anemone this way you can have a little color more often.

Look forward to seeing the after images!