Not really sure of your planting zone, but hydrangea would seem to be a good solution.

i asked... how you plan to actually do the pruning.. ??? without water in the pool i am having a hard time conceptualizing... the dry workspace ... will you be walking along the stone cap with pruning shears.. and a floating garbage pail to put all the cuttings in??? you can come up with ideas about all different kinds of plants.. that is not the issue i cant solve ... the issue is.. how do you care for them.. short of being a fairy with wings .. tinkerbell .. who can hover on the other side of the wrought iron fence ... in mid air .... i see some kind of something coming thru the square caps.. what is to be installed there.. lights??? .. if so.. you wont be doing a Karl Wallenda along the rest of the caps ... see link ... and that best defines how i dont understand how you would prune and care for plants of various types .... and that is the limitation that concerns me ... ken Here is a link that might be useful: link

Typically, back yard neighbor's who are outside using their back yards do not want to see other neighbors who are also outside using theirs, or be seen by them. Serviceberry would make good backdrop trees, whereas the existing trees in the lawn will be shade trees and, presumably, limbed up, not being good for privacy. BTW, it is common that plantings for screening exist near property lines and that the foliage of these planting grow into the adjacent yards. That's a fact of life because everyone does not live on a 5-acre estate, but most everyone wants some degree of privacy from their neighbors. Privacy screens installed by one neighbor usually, simultaneously benefit the adjacent neighbor, which is why people do not mind this condition much, or at all, so long as the plants are not obnoxious or invasive.

@Yardvaark It's my partner's house. I don't think he thought this through to the very end when he started. His only goal was the "cut back the hill for more flat lawn in the back and use the sand to make the lawn level up front". I can take more pics when I'm there next time. I wish I had before pics so you could see the difference. This used to be a very closed-in mostly grassless pine and hemlock grove with a house plopped down on the edge of it. There were trees everywhere, very little sun reached the back (west) or side (north) of the house. Once he got the trees removed (some of the big 100-200 yr old pines were professionally removed, but we've done the hemlocks ourselves), the grass started growing, and once he saw he could have a real lawn, he hasn't stopping digging and moving the sand around. It started as digging out the stumps, and it's been non-stop digging anything that can be dug and spreading the sand in other places for over 4 years. It's only now that he's gotten close to the line of driveway reflectors that he used to mark where he wanted to stop digging back to that he's considered that a sheer drop off of sand under an azalea that is floating at eye level (with roots sticking out at chest level) isn't going to be easy to landscape. He saved the last couple of Hemlock trunks we took down so he has 6-7' logs he can possibly use to make a retaining wall, but I'm getting the twitchy (skeptical) eye looking at them. I'll take better pics next time I'm there. It's truly a landscaping nightmare. He will not invest much money into landscaping. It took 5 years of deliberation to buy the birch trees he'd wanted all that time, and only then because they were discounted at the end of the season (the road is called Birch Lane). He refuses to buy any annuals (waste of money, he says). Perennials need to be 95% sure to survive and come back, so he will only buy 1 or 2 plants a year until he's sure it will come back (so creeping phlox, virginia spiderwort, chives, chocolate mint, periwinkle (which is invasive but he doesn't care), hostas, solomon's seal, lily of the valley, azaleas, rhododendrons, maidenhair ferns, lilacs, and day lilies are the only proven plants so far - most of them shade plants and he's reducing the shade more and more). The exception is if I can find it for free (like on freecycle) and the western red cedars he planted for privacy screening after we removed the hemlocks from the back. Something that can be started from seed would be great since seed is cheap, I can just buy it for him and he's not stressed out about wasting his money on things that won't be successful - with a caveat that he's also reluctant to water things as much as they really need because "water isn't free". So, that's what I'm working with, and I admit I am pretty over my head right now because I have no idea how to deal with the sand. The erosion problem is, in my mind, a serious one, but apparently the town doesn't mind that part of his hill (in the front, the road frontage, not back here where this project is) washes out into the road when it rains hard, so we're concentrating on making the back yard nicer since the front hill is basically impossible to fix without a huge investment in retaining wall (which he will not do - he already invested $30,000 in replacing the windows and he doesn't plan to keep this house since it is just his first starter home). I'll get more pics!