IF you always want to rotate 90 degrees, why worry about axis values? You only need to consider the sign (to decide whether to rotate clockwise or counter-clockwise)
Set Rotation is a node that is available in the blueprint. Simply start typing it and you will get it. Put self as the target (default). The new rotation calculated by adding (or subtracting) 90 degrees should be the input. To transition smoothly, you can use a Lerp node and feed its output to the Set Rotation node.
If you still cann’t get it right, I will try to create a blueprint myself and put it here (will take 10 hours though - I not at my place right now)