I love, love, love this shrub, it's adorable when in bud and stunning in full bloom. The light green leaves are pretty for the rest of summer. However, I'm not sure it will meet your criteria, given your zone. Mine is in full bloom right now in Zone 6 & about now the hosta are coming up. Maybe it will leaf out later in your zone?? Although, if the one at your garden center is already leafed out now it would be OK. Mine has never been salted so I can't speak to that.

I love mine too, BUT, it is extremely sensitive to late frosts and must be covered whenever one threatens or the foliage burns and you lose the bloom.

My Deutzia Chardonnay Pearls is fully leafed out right now (leaves started expanding about a week ago) and has good sized buds, so it is a bit ahead of the hostas, but only by a couple of weeks. It does get a bit of branch tip dieback if it gets really cold before there is snow to cover it, so I think that like for me Chardonnay Pearls will be near borderline for hardiness for you. My CP is in a bed on the east side of the house, so morning is full sun and afternoon is bright full shade. I'll see if I can a photo of it sometime tomorrow.

Here are a couple of photos of my Deutzia CP which grows on the east side of my house. The first one is when it was in bright shade most of the day due a large maple to the SSE,mostly just out of the photo, though there are a few branches just showing on the left. CP is on the right side of the photo, just to the left of the metal obelisk. The color was greener than it might have been due to the shade and so doesn't show up as much as it might have.
{{gwi:254597}}From Deutzia Chardonnay Pearls

The second one was taken this week on May 23, and I know that it leafs out a bit before the hosta, but am not sure how much earlier. The tree was removed last spring, and you can see that it gets more chartreuse now than it did with more shade. This part of the bed gets morning sun and bright shade the rest of the day now. You can also see the results of the tip dieback; prior to pruning it looks a bit uneven.
{{gwi:254598}}From Deutzia Chardonnay Pearls