Multipurpose CameraRig - Smooth Movement - how to achieve?

Dear Unity-Community,

does anyone know how to add smooth movement to the camera?
(I am using the standard camera asset / Multipurpose camera rig attached to a RigidBodyFPS Controller with an additional script to move up and down)

My goal is to have a delay or fade in/fade out of let´s say 3 seconds when moving the mouse in any direction as well as when using W, A, S, D, (+ Q & E for up and down) so the movement doesn´t stop abrupt when stopping the mouse or any key.

I would very much appreciate any help regarding this!!

has anyone an idea how to achieve that?
I saw this post:

it´s pretty exactly what I need, but I am not familiar with trapezoidal acceleration

who can make my day by providing some more info about this?!?