Privacy and noise protection on a slope through poplar caps or fir trees?

By the way, my plan now is to pile up leftover soil from earthworks to raise the ground by about 10-20cm and to level out unevenness. After that, a layer of stones will be placed on top, which I will hold in position towards the slope with iron rods. Above that, I will build a wooden soundproof fence: thick, overlapping, vertical boards should effectively reduce noise. On top of that, a horizontally lying board as a "roof." Something like this, but probably a height of 1.0m to 1.50m will also suffice: The stones serve as protection against moisture at the bottom; on top, a horizontally lying board as a "roof" will be placed. The whole structure will be supported by individual posts, but where possible, the tree trunks are supposed to hold the fence. I am just not sure whether the trees will keep growing after I cut them down? I don’t want the fence to be lifted upwards. In that case, I would completely cut the trees at about 3-4 meters above the ground level, not just break them off. The fence will already provide privacy. I am happy to receive comments or better suggestions.

A considerable wind load will act on the fence. Therefore, the posts of the fence must be "properly" founded. As a rule, it is sufficient to create a frost-proof foundation hole (60 to 80 cm deep), into which the posts are then concreted. How it looks on your site cannot be said from a distance. If in doubt, you will need a structural engineer. Yes, really, the fence is not supposed to become self-walking and kill someone.

Possibly, a development plan, the neighborhood law, or other local regulations regarding enclosures and building law must be observed. You surely have that on your radar.

I wouldn’t nail the fence to the trees; I would set up the fence independently of the trees. Just like in your photo, it looks great!

What bothers you about the trees that they are supposed to be cut?

The posts will be concreted in, yes. I wouldn’t hire a structural engineer; as mentioned, the fence will not be as high as in the picture and there is still planting and bushes all around, so there isn’t much vulnerable area. The main goal is an inexpensive solution.

The development plan etc. is all settled.

The trees should be cut back, ...

on the one hand to keep maintenance at a tolerable level, on the other hand not to impair the western sun too much.

Since we are already shaded by a multi-family house to the south, at least the sun should come in from the west. You can also see in the pictures in the first post that the trees hang quite strongly over the sidewalk, and poplars are not exactly famous for their stability. Therefore, I want to cut them 1-2m above the top edge of the fence, so at about 4-5m growth height.