Also looking at your API, I used a vector type a lot for defining things like planes, so where you have sketch("xz",func) mines a little more like plane(vec).sketch(). How are you handling sketching directly on arbitrary planes or faces etc?
The user can create planes with plane() command e.g plane("xy") or plane("xy", { offset: 10 }); but I have added some shortcuts for common planes (https://fluidcad.io/docs/guides/sketching/introduction#sketc...)
you can also sketch on faces sketch(extrusion.endFace(), ...) which will extract the plane from face and put the starting point on the center for convinience.
const in1 = draw.rectangle(size1, size1).fillet(2)
const in2 = draw.origin([0,0,height1]).rectangle(size2, size2).fillet(2)
const result = make.loft([in1, in2])