My idea FWIW=
Divide the area with narrow paths going from corner to corner, maybe an extra accent stone in the center. Makes triangles.
Four plants, one in each triangle. Each plant will be three or five to start with depending on the size of the starter. Mulch around.
Tallest plant against the yellow wall- monarda
side by side- coneflower, Butterfly weed (tuberosa) OR a perennial with spiky flower, like a Veronica?
front- pentas
Three are perennial, pentas are annual. This could be done in pinks/purple/white tones. Decent nectar plants, don't know if any but the butterfly weed are good for caterpillars.
Or bronze fennel in the tallest spot- food for swallowtails, great flower for many small pollinators. Tall, airy, bronze foliage would look nice against the yellow.
Just one plan. I think dividing with a narrow path or bricks lends a neatness factor and helps you keep the plants confined.
