local/world rotation of the Universal Transform Gizmo in Editor

What sort of dark perfect magic is the Universal Transform Gizmo applying to do such local rotation?
I’ve tried every combination of FRotator Lerp, FInterpTo, RInterpTo, Quaternion Lerp, Slerp,
even attempted cracking quaternion math to apply the same smooth alignment of values.

Here’s the scenario:
An object that doesn’t have 0 X-axis (roll)
Smoothly interpolating around the Z-axis (yaw)

The general calculation is: roll - (targetyaw / startingroll)
however smoothly, which FRotator Lerp, FInterpTo cannot do,
and FQuat Slerp, Lerp, doesn’t calculate.

I don’t really understand the question. What are you trying to do here?

I think you might be able to achieve your goal with AddWorldRotation - AddWorldRotation | Unreal Engine Documentation