The only way to prevent changing thrust issues and still have a soft mount is to use a nose ring. This is the norm in pattern where the mounts are used to reduce noise and to extend the life of the airframe and gear, generally a Hyde mount with a nose ring (various makers.) The soft mount allows mainly axial absorbtion, but will allow the thrust line to move. The nose ring stops the thrust line from moving and keeps all absorption of the mount axial. A stiffer mount will help reduce the changing thrust line, but ultimately the solution is to use a nose ring.