Sometimes a reply is not sufficient to close a question.
Anyway, I don’t have code, but you could try this approach:
- make the model 2D – by applying mesh transformations into the geometry and then clearing one of the coordinates of each vertex (e.g. z=0)
- extract the individual triangles (they should all be co-planar) and convert them to shapes
- combine all shapes into one shape with clipper2-js