[Problem masking]

Problem masking

Hello, I am describing my problem here and hope for a solution:

I have a picture, a drawing with a signature. The signature is not easy to read, so I created a new layer and redrawn the signature by hand.
This layer lies above the image layer. This works fine.

Now the problem: I want the signature of the original picture to show through. That's why I masked the drawing in the area of the signature. I can use it to completely hide the signature.
But if I want a transparent, shimmering effect on the signature by reducing the opacity of the mask, the rest of the drawing is also reduced. what am I doing wrong here?

Kind regards and thanks for the help!