You already have several options for drawing “smooth curves”, for example Segment editor you can use Surface cut effect (you need to provide at least one point in a different plane) or use Paint effect with a very large diameter sphere brush.
However, we have a much better tool for this now. We have recently released the new Baffle planner module that allows editing of 3D surfaces both in slice views and in 3D. I’ve created a demo video to show how it works for this use case:
