For 1 cm changes in handlebar height, your changes in setback seem rather steep. On the other hand, you're starting from a fairly high position and just scratching the surface at getting somewhat low. Also, your starting point, 7.5 cm behind the BB, is rather forward unless you're quite short or your saddle is unusually long in the nose. What is your pedaling technique like--are you pedaling circles or primarily pushing down?

Any change in position requires acclimatization for both supporting the core and turning the cranks. Climbing steep grades will probably be the activity most resistant to changes in position because it's intense, it involves the entire body, and the entire body has been trained to do it the way you've been doing it. Doing it in a different position takes some re-learning. Better to test changes in technique over flatter terrain, and gradually.

My advice is, handlebar drop is not as important as back angle and reach to the handlebar. Aim for a 40-50 degree back angle, because that's where the power is, and a 90 degree angle from the back to the upper arm, because that's where you can pull on the bar for more power. And if you're not there yet, go in steps, and give your body time to learn the new technique before you test it.