Eventually got one of my models into Paint3d after writing my own exporter, but it looks awful. The format does not have an ability to process surface normals, plus it expects only solids suitable for 3D printing.
I had to import it into 3DBuilder first so it could ‘solidify’ everything.
The model came to 30MB, and it wouldn’t upload to remix3d either.
So it seems Paint3D isn’t for 3D modeling after all, only 3D printing, which is a shame because I liked the spray can feature for painting models.