AdamDZ

Looks like too much weight on your hands or wrong angles somewhere. I second the notion that you should see someone who knows how to fit a bike properly. Perhaps you need more shock absorption, i.e. thicker tires too?

However, I had similar problems when I owned road bikes. No matter how much padding I'd used or how the bars were adjusted, I even tried different length stems: I was always getting that kind of pain at the base of my thumbs, when riding for too long on the hoods. Including a shop-fitted carbon bike.

My experience was totally opposite to this:

I guess, we're all different. There are no rules that apply to all and no solutions that solve problems the same way for everyone. I grew up riding with flat bars. I was over 30 y.o. when I rode drop bars for the first time, so perhaps my muscles, joints and tendons are developed differently. Drop bars made me suffer most of the time. I was getting elbow and forearm pain too on drop bars, the position didnd't feel natural, my arms felt twisted.

To be fair, poorly padded straight bars would also cause pain during bumpy rides, so I always use gel grips or Ergon grips.

Don't mess with your saddle. It must be set up just right. Too low and your knees will start to hurt. Too high and the tendons underneath your knees will give you trouble. You can experiment with the horizontal saddle position (move it forward or backwards), the stem lenght and angle and the handlebar position, but leave the saddle height as it should be.

Also check the drops angle. If the hoods are too low or too high they can give you trouble too.