We're happy to share some ideas, but could use a bit more information first. How much direct summer sun does the site receive? And does the soil drain well, skew dry (given the heat radiating from the road in summer), or tend to drain poorly due to runoff or compacted soil? Astilbe thrive in semi-shade and damp conditions, so it's hard to tell from the description what the growing conditions are like.

What approximate size would you like the shrubs to grow to, without relying on pruning to restrain growth?

Miri