You will need to pass the index of the pressed button to the OnButton and SmoothRotate functions. To do this I would recommend taking a look at this code snippet:
fortnite-wrapping-subscribe-to-pass-additional-data-to-listeners
This will allow you to change your OnButton function to
OnButton(Agent : agent, ButtonIndex : int)
and then you can subscribe to the InteractedWithEvent for each button in the following way
for (Index -> Button : ButtonList):
Button.InteractedWithEvent.SubscribeAgent(OnButton, Index)
Now you can use that index in OnButton to get the Prop and then call SmoothRotate with that prop.
I hope this helps, Good luck 