What does force feedback strength multiplier actually do?

Force feedback reads the torques simulated at the steering column, and scales them when they are fed into your USB steering wheel. The amount of scaling is controlled by this multiplier.

For drifting, it's about suspension design. An old vehicle with boaty suspension will be harder to drift, as its suspension design is barely communicative compared to a more modern sports car. Vehicle setup also has an effect on the forces arriving at the virtual steering rack.

You will want a fast communication with the steering wheel for FFB to work better, so try to keep your framerate above 60FPS, 100 or more preferably.