Someone told me, my previous method 👆of testing isn't meant for measuring downweight.
So I repeated the test differently:

This time I added weight until the keys started to move ever so slightly.
I lifted the keys again, but the weight just was enough to push the key a little bit down.
The outcome is similar:
Near the fallboard, the black keys are ok-ish but the white ones are just hard to control.
Seems the letoff-notch on the black keys needs (relative) more downforce than the one of the white keys.