The axis keybinding for movement will have far more control over the direct input of keys.
Direct input by nature of the feature is not accurate. Its like trying to play a driving game with left right down up keys. you are missing the "smooth" control you would get from a joystick or game pad.
DSS should have an option to use the mouse instead which should have more fine motor control then a direct input from the keyboard.
But you are correct, Elite does not have any further options on direct input options such as fine tuning how drastic the change is when a key is pressed.