The wall is mottled. What to do? New construction

I think rather that the direction of brushing was not followed here. And cheap hardware store paint?

Place furniture in front??

The window is really amazing, and the little triangle.. who plans something like that. What paint was used? Surface not evenly pretreated? DIY or painter?

Attached are more pictures.
Pufas Grundierweis GP 5 was used as a primer. Herbol RF paint was used as the final coat. Both are actually painter's paints from the specialist trade.

Unfortunately, it is difficult to see in the pictures.
The staircase landing also appears narrower in the pictures than it actually is. The corner at the top left is caused by our Frisian gable. The wall has already been heavily primed and painted twice. The wall was already evenly white before priming.

We sanded the plaster in advance with a sanding machine. The plaster "reacted" differently to the sanding. In some places it was smoothly removed, and in other places you can clearly see sanding marks. That is why we chose the Herbol RF, which has good properties and still a high filler content.
Maybe the appearance also comes from the fact that the substrate is different in terms of surface (partly rough, partly smooth).

P.S. Painting is always done in the same direction per wall.

In my eyes, it is a problem of light incidence. I hardly believe that it can be painted better. Probably just have to live with it like that and that's that?

We suspect the same. But I thought it could still be improved. We painted 2 more rooms yesterday. It also occurred there. The light incidence is similar there. You only see it when looking at the wall from the side and against the light. It is annoying us, though.