Screw wooden board onto concrete wall

Yes, 10 cm is not really a problem, you still have more surface that adheres. I wouldn't necessarily stand on it to attach the curtain rod.

But just make it 50 cm and a proper window seat with drawers underneath. That's how we're currently planning it for my wife's hobby room. Of course, you'll also add supporting side walls.

For the windows on the dormers, we have the following conditions.

The windowsill is about 2.20 meters long and 30 cm wide, with about 18 cm protruding into the room. In other words, only about 12 cm rest on a beam and a drywall.

We then screwed three shelf bracket angles to the vertical beam, and the windowsill rests on them.

It is not screwed in anywhere but is slightly clamped under the window frame.

In the winter months, there are a total of four 80 cm flower boxes placed there. These have quite a bit of weight, especially when the soil is wet, but the brackets hold up.

Perhaps this could be a solution for your situation.