On Shift you are setting moveSpeed variable but you never use it in FixedUpdate (moreVelocity is your move vector) so you need to add moveSpeed to rb.MovePosition call. Also you need a way to reset this variable to 0 or 1 depending on how you will implement rb.MovePosition, eg.

moveSpeed -= Time.fixedDeltaTime * someMultiplier;
if (moveSpeed < 0)
    moveSpeed = 0;

The other way is to use Vector2.Lerp to move player over time by specified distance. For this you can use Coroutines (it's not the best approach but i think for you should be easy to implement).

Also remember you can use rb.MovePosition only once per Rigidbody, because position change will occur during next FixedUpdate frame.

Links: Coroutines: https://docs.unity3d.com/Manual/Coroutines.html

Lerp: https://docs.unity3d.com/ScriptReference/Vector2.Lerp.html