Because the air is thinner, the plane has to fly faster to produce enough lift at a given angle of attack. I would try a smaller higher pitch prop that is sized to give a higher RPM. I'm not sure where the ST 51 produces peak horsepower but I'll bet it is closer to 12,000 RPM than to 10,500 RPM.

At such a high altitude you may have to use an even larger engine (or smaller plane) to get the performance you want.