How about a couple of ornamental trees that bloom, like vitex? That's what I used and they grow quickly but want regular trimming. Under them I was able grow almost anything that didn't require dense shade.
Japanese maples are also good as well as butterfly (buddleia) bushes.
The vitex didn't mind me planting right up to about 6" from the multi-trunk and by hottest part of summer it had developed a broad and arching canopy of growth.
For immediate remedy for this spring bloom of the astilbe why don't you just buy a potted , leafy evergreen tree and set it among the astilbe . Something like a euonymous or leucothoe. You can plant it later when you've move the astilbe to a north or east facing bright light but not direct sun area.
Tuck in some Bloody Crane geraniums. Some iris might be nice. More peonies..different colors. You might want to move your Lenten rose plants when you move the astilbe if you think they'll perform better with indirect light.