For Cascade, you can bind to OnSystemFinished, but such an event dispatch does not exist for Niagara, as far as I can tell.
In 4.25 it exists so if nothing else this was fixed.
As for myself I actually need an “OnSystemStarted” or something because the effects usually start playing after a frame or two, making a ribbon trail start about half a meter offset from the weapon barrel. The general idea is to not move the system by default and only start moving it After it has started playing, lerping it to the intended location in a smoothed manner.
This would eliminated the gap at the beginning while it would be virtually unnoticable - but I have no way of telling when the system has started operating… 