The photo is a little fuzzy so can't tell for sure, but you might consider allowing the burning bushes at the end of the house to grow into tree forms if it wouldn't conflict with any windows. And then underplanting them in the future. For below the front window you might consider one of the Japanese Spiraea cultivars. At the back of the perennials, if there is enough light, you might consider including Siberian Iris 'Caesar's Brother'. It has long lasting nice foliage in addition to the great blooms and general plant vigor without being invasive. Potentilla somewhere? Peonies? A tree Peony?

I like the idea of some spirea there. Will they get tall enough to fill the area below the window? That's an interesting idea about the burning bushes but there is a bay window on the right and they are already too high and blocking some of that. I plan to trim them down after they drop their leaves.

"Will they [Spiraea] get tall enough to fill the area below the window?" There is a multitude of height choices available. The right height would minimize the need for frequent trimming.

Okay, I just measured and the picture window is exactly 36" off the ground. I just discovered Thor Spirea, which grows to 2-3' so that could work. I love the fall color these have right now. Or would those be a better fit to run along the path and plant something else under the window?

The Spiraea would be the tallest planting that area. Between it and path, in general, should be something much lower.