Here’s the thing. FInterp works by outputting a number greater than the current input, and <= the target input; driven by the Delta Time input and the Interp Speed input. As a result, to reach the target number, FInterp needs to be called multiple times(e.g. from the Tick function) with up-to-date values. I.e. the output from the previous call should be the Current input on this call.
Unfortunately, the mouse button inputs are one-time calls. Ergo, the setup you posted will not work as you want.
I would suggest trying 3dgo’s first screenshot, as that would be how I would set it up.