So new discovery after a little experimenting.
I did say some pages back that the Cascade looks smooth, and after playnig with it, I found its the smoothest in the game, and why is that? my theory was "Maybe because the camera is locked to the boat" you see, everything else in Planet Coaster has this weird camera where the camera starts to turn because the train car starts to, so I wonder to myself "Is there anything else in game that has a camera locked to a car?" and there is! The Spinning Coaster.
So, I make a spinning coaster, I turn off restrictions, I do a curved downhill drop, followed by a corkscrew, I don't use the 4 meter technique, I just use the normal smoothing tool, and BAM, its smooth, no problem, I advise others to try this too, because this may just be a camera issues and not the builder itself (well, apart from custom heart-lines)
Edit: it is still janky, but not in the same way, let me know your results