Thanks for the response! I was able to use transform.basis_xform by doing the following:
input_vector = transform.basis_xform(input_vector).rotated($Sprite.rotation)
(had to use my sprite's rotation since it is rotated 90 degrees so that using
look_at to my global mouse position works correctly)
But this seemed to do the trick! Thanks for the catch on
move_and_slide, that's very helpful.