Open the ABP_BaseHoldableAnimLayer blueprint then open the UpdateTargetRightHandIK function and copy the images below. You will already have the GetSightHandIK collapsed node in this function, reuse that node.

Then open the UpdateHandIKData function and add the nodes shown in the image below.

You can now change the Aim and Unaim Transition Speed setting variables in any of the holdable blueprint anim layer blueprints in the Class Defaults, higher values will make the transition faster.