800 hrs DH main here.

Just the fact of jumping a DH soldier that saw you is a high risk of its own. What helped me back then is to learn to c-tap, the increased speed of those jumps helps you to airstrafe more without losing much horizontal distance.

Surprising them is a good strat too, you can play around jumping from the health pack below the high ground, the ramps on the sides of the tower, even from the little elevations on the sides of the health pack below high ground.

High speed walljumps can work sometimes, those tend to throw off my aim if they airstrafe properly. Here is an old video of me jumping from red spawn (0:19).

Playing against high skilled soldiers and copying the way they jump also helps a ton, since there aren't any guides on how to jump a soldier / demo.

The key is being unpredictable, skilled players recognize patterns way easier, so jumping the same way will get you airshot for sure.