Rugged perennial for "wet feet", part-shade area in Zone 6 ?

The ground is shale and dense clay here on a mountainside, so my small herb, lettuce and rhubarb garden lives in an 8' x 16' raised bed. Along one of the 16' sides, my husband dug out a 3'-wide strip - not a raised bed, but into the ground - for a perennial border, which is doing quite nicely the past four years, except at the bottom end. The ground slopes just enough that heavy rain accumulates at the low end. The perc rate of this soil is very slow, so the clay/mud wall at the end of the bed is backing up the water and killing the plants at that end. So far, a Hylotelephium "Autumn Joy" (fka Sedum) and a little mound of dianthus rotted out and failed to re-emerge. At the top end and the middle of the bed everything's fine, with coreopsis, astilbe, columbine, dianthus, lamb's ears, hyacinths, species tulips and peonies coming back and proliferating.
We're not going to do more digging in this area. Later in the season, nasturtiums and four o'clocks are quite happy at the wet end. But I'd like to have a perennial or two at the wet end that come out on their own, and survive, so the bed doesn't look so lopsided in the early spring.
So far, I thought of digging up some of the Virginia bluebells and bloodroot plants that I have at woods' edge (they prosper, even in areas where the water stands after a heavy rain) and installing them there. Any other ideas of something that might survive these advense conditions? The bed gets about 3 hours of direct sun in the morning, then dappled light the rest of the day. Zone 6.
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