Construct retaining wall / slope stabilization cost-effectively

For a wall that is supported on one side and free-standing on the other, is "vertical" recommended???

Yup. Because basically, with 36.5 formwork stones and the reinforcement that must be integrated, L-elements are practically cast on site.

Without reinforcement, the maximum height is significantly lower (see also the value in the picture)

Funny. The manufacturer differentiates the backfill only into with and without traffic load, but regardless of their viscosity.

Because basically, with 36.5 formwork stones and the reinforcement that has to be integrated, L-elements are practically cast on site.

However, I do not follow how you see L-stones practically forming through the reinforcement.

Since they are fir trees, pruning looks very poor. The firs only block the distant view and since they are not dense at the bottom, they also do not provide any privacy. Therefore, they are to be removed and taller shrubs should form the privacy screen.

If I were to keep the trees and only fill up the property, I would have a much larger slope towards the back without appropriate reinforcement to the neighbor.

If I were to leave the trees and only fill the property,

I would not fill anything in front of the privacy screen in the building line, but rather leave the existing terrain.

I wouldn't add anything in front of the privacy screen in the building strip, but rather leave the natural terrain as it is.

I don't understand what you mean

I would also consider whether an underplanting of the fir trees would be sufficient. For privacy at the lower level. How important is the distant view to you?