As others as mentioned above, piston engines really struggle at high altitudes due to thinner air. In real life when operating from such altitudes, I will check for what’s know as “density altitude” and there is a limit to it. Many AFMs (Aircraft Flight Manuals) include a maximum altitude and temperature for takeoff/landing to which aeroplane can be operated safely, If the ambient conditions exceed those limitations than performance is not guaranteed in real life.
Now I am not sure how accurate modelling is in MSFS but in order to improve takeoff performance, You could possibly decrease your payload and fuel and see if that helps. Milviz from my FSX/P3D experience seems to be quite realistic