Problem with range of brush

I have this situation. I created a 3d object outline/contour, used a plane and boolean modifier for this.
The outline/contour is linked to its duplicate. Where it displays without modifier.
An image with pattern displays below the outline. Here You have screenshots, which shows the situations

All of this has been done to match the outline/contour with the red spot in the image. I use sculpt brush for this.
Editing with a sculpt brush on the outline/contour works almost fine.

The problem is that if the brush edits only the contour with no problem, it actually selects a lot more of the object. Brush reach is full height. Is it possible to do so that when editing the contour, the brush changes only where the contour is?

I wish the brush would only work on this thin piece of outline/contour.
Maybe someone has any idea how to change the brush range?
Bee grateful for your help.