Basically all that’s available is adding more points to the collision polygons. Or maybe make a number of sprites for various curves and such using as many collision polygon points as needed to make it smooth enough. Then build your terrain with a bunch of those.
Another idea is to design your terrain with something else, and then use sprites as lines to define the outline.
I’ve used a vector drawing program before and saved to its simplest file format and parsed that to get line segements making up the shapes.
Another idea is to make your own in game editor that you could disable later. You could use qarp() and cubic() to do Bézier curves that you then split that up into lines. At least it beats manually placing individual points.