I’ve been trying to work with built-ins rather than approximating the function iteratively, on the theory that Processing can do that better than I can, and after messing around with a demo program in desmos to get the right specification for the Bezier control point coordinates, I can create the shape I want with a Bezier and a reflection of its control points in Processing. So - that’s nice.
But in taking the next step and adapting that definition to PShape I’ve run into the ‘bezier is flaky inside PShape’ problem - bezierVertex() draws straight lines, not a curved line? 
- but that’s ok as I can either add an argument to createShape that fixes it for the default renderer, or change the renderer. It’s an open bug so there isn’t a solution that works for both situations.