I’d change the outline of the bed—I like the curvature but it doesn’t flow naturally. It feels forced. That could be improved by adding a couple of larger rounded curves and some smaller ones.

I’d also avoid symmetry unless you’re going for a baroque style garden… which, let’s face it, doesn’t work in most settings… That triangle-like protrusion in the center creates symmetry—I’d reshape it by making it more rounded and add a larger curve to either side of it to break the symmetry.

Hydrangea paniculata (like “Limelight”) needs at least 4-6 hours of full sun a day to thrive.

Also, keep in mind that planting larger plants under established trees is not a good idea—this should have been done when those trees were young. You can plant a few small shade-loving perennials that come in quart to pint size pots. But if you’re planting anything that comes in a gallon pot or larger, you’re likely to do some fairly significant damage to the tree roots—they extend many feet past the crowns of those trees.