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There are many options available to you! These articles will offer some suggestions:

https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-6/bushes-for-zone-6-gardens.htm

https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-6/zone-6-hedge-plants.htm

These articles will offer more information on growing in zone 6:

https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-6

Unfortunately, your photos did not come through. I am unable to assess the situation.

These articles will help in the meantime:

https://www.gardeningknowhow.com/ornamental/trees/sand-cherry

https://www.gardeningknowhow.com/plant-problems/pests/pesticides

It wouldn't hurt to prune out a few of the scraggliest ones now, but save any major pruning for late winter. This article should help:

https://www.gardeningknowhow.com/ornamental/trees/sand-cherry/pruning-plum-leaf-sand-cherry.htm

I suspect it is related to the actions you took that day, and it could be that the roots were damaged when the annuals were planted. Was much digging involved? Either that or it was overfertilized. Go ahead and water well to flush out excess fertilizer if you think that may have happened. If the roots were damaged, the extra water will help it recover. This article should help:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/fertilizer-burn-in-gardens.htm

Sand plums typically reach 8 feet tall and wide, so it may outgrow that area soon. Once it recovers, you might consider moving it. Here is more on its care:

https://www.gardeningknowhow.com/ornamental/trees/sand-cherry/purple-leaf-sandcherry-info.htm

This will be done in early fall. As long as temperatures are manageable. All you have to do is dig around it making sure to get plenty of roots. If you accidentally cut too close you will cut off too many roots for it to be able to grow its own in time. Just transplant where the soil level is up to where it was before Keep the soil slightly moist until it is established. Wet or dry soil will kill it before it establishes. It needs to be somewhere in the middle.

It is worth keeping in mind that it is likely grafted. Anything below the graft point will not be like the top. Anything that comes from the roots is likely a host species like Prunus cerasifera. The rootstock is usually hardier than the top of the graft, but doesn't usually resemble the top of the tree at maturity.