Don't want to burst your bubble, but all trees drop their leaves, including conifers. I have a row of some sort of conifer that has little round seed pods. In addition to the leaves that fall naturally seemingly all year round, they are also incredibly messy whenever it is windy and gutter cleaning is a regular chore. The hard round seed pods are a real joy to walk on - hurt muchly if you are bare foot, very easy to turn your ankle if wearing shoes. And if that isn't enough, in spring the trees flower. And discharge huge amounts of pollen - yellow dust which covers everything. And do remember that these trees are not going to stop growing at 3-4 metres. If it were my swimming pool, I think I would prefer a deciduous tree that dropped it's bundle once a year. If you have a neighbour on the other side of the fence, do remember that these trees are also going to affect his/her gardening in the area of the trees.

A lot of the clumping bamboos (most, I believe), that are hardy in zones 7 and 8 are subtropical mountain species. Which means..., they are not as hardy to extreme cold or too extreme heat as that notorious running genus, Phyllostachys (you don't wanna go there!). F. rufa seems the most commonly available in the Mid Atlantic area. It is a nice species and does put up well with our hot nights (a potential issue with many of the mountain species). It has seen temps from 4F to 109 without any burning. It is not cheap, and you will not get a privacy wall overnight, but will, eventually, if it likes its positioning. Some shade from the hottest sun is definitely a big advantage. It is well-behaved and will not be invasive, but will form a dense grove. We have Fargesia rufa (Green Panda Bamboo). Started with little pots and every year would divide each plant to lengthen 'the wall'. Growing area gets morning sun and dappled midday. It is never watered nor fed.

Are you willing to prune annually to keep plants to the size you choose, either height or width? When you say you want height, are you thinking high enough so that someone can't see over it, or so that the 8 story apartment across the street can't see into the yard? It sounds like the area where the hedge will be is in full sun, not shaded at all. This is an advantage since it means you are more likely to have leaves densely clothing the hedge top to bottom. How is wind exposure? Think about whether you want all of one kind of plant, which looks more formal, but will be a problem if a pest or disease infests your hedge or even if a plant dies, trying to match the older plants; or if you want to have a mixed species hedge, using repetition of groups of plants. It looks less formal, but will be more resilient in the face of problems and may offer more variety in the way of food and habitat for birds. IME if you have a good quality foundation, the roots from a hedge won't be a problem. I am right on the edge of 5a/4b, and IME, if you want bamboo, you will want to be OK with one that might get killed back in particularly harsh winters, like the last one, though mine hasn't died back most years. Mine (one of the hardiest Fargesia, it gets only to about 6' here ) also doesn't do particularly well with staying upright with heavy wet snow, so you may lose some privacy in the winter. Others from farther south or with different varieties may have different experiences, but that is my experience a bit north of Concord, NH. Arborvitae is fine if you don't have deer. Depending on the species and cultivar you choose, they can range from 8' or 10' to 40' and have a color range that includes yellows, greens, and some lightly frosted or variegated. Some types are more likely to grow multiple stems, and those are more likely to split apart in heavy snow or ice, so if that is an issue for your yard, you may want to choose carefully. There are also some narrow hollies and junipers. Skyrocket juniper is a form of J. scopulorum which is from the western US, so is more prone to disease in humid areas. Sky Pencil holly is a form of Ilex crenata and looks a bit like a narrow boxwood - the leaves aren't prickly. There are narrow yews such as Hicks yews, but yews are particularly beloved by deer, so if you have deer, you don't want yews. I don't have any experience with the narrow forms of any of these plants, so perhaps others can add info about how they fare in the landscape. There are also some fastigate small to mid-sized trees such as Japanese and other maples (Acer), Sweet gum (Liquidambar), Hornbeam (Carpinus) as well as a number of fastigate/columnar evergreens such as varieties of pine, fir, and spruce. Many of the evergreens will grow to tree height, though not all. In general these will tend to be more expensive since most require grafting to achieve the narrow form.

Wax myrtle could be a good option in your area if soil is neutral or acidic. Many culinary/(folk?) medicinal uses and I don't recall hearing of any toxicity issues for children or pets. Nursery person should be able to confirm. Loropetalum is another that might work. Most of the newer named cultivars will be smaller than what you're looking for. But the unnamed varieties found at the big box stores can usually make a large shrub or small tree. Major ice storms can be hard on these, but are pretty good about recovering.