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You do not need full sphere’s, in fact when scanning from a single PoV there is almost never more than half a sphere captured from most scanners. In the past I have used best alignment across multiple spheres from different sensors and positions with typically <40% of each sphere visible from any on...

Just checking, are you using the distance to primitive function? That won't have any "ripple" because it is calculated on the sphere definition rather than the drawing detail level like the distance to mesh function.

You would want to do it when both the cloud and the polyline vertices are flat with z=0.
So immediately after creating the contour polylines that's when you should copy the z coordinate SF from the original cloud

This may be worth adding as a standard option to contour extraction maybe "3d contour" or "in place envelope"... not sure on the name, but I can see the use of this.

I had good success following what I told you, zeroing out the Z value getting the contours then restoring the Z values. only extra step I didn't mention was to interpolate the original clouds SF values to the vertices of the extracted contours and restore the Z SF to the vertices as well to get the ...

you could extract z coord to a scalar field for storage, then set all Z values to 0, then extract envelope, then measure distance to primitive, then restore the Z coord from the scalar saved earlier

Also if you want the points near the envelope, you can use the Cloud to Primitive distance tool to evaluate each point to the polyline that represents the envelope
You can then filter the cloud based on that distance.

It should not matter if they are not on the same Z level, I think the confusion is terminology, in the cross section tool under extract contours, the thing you are asking for is called "Extract envelope" rather than extract contours.

These micro optimizations are very challenging, when trying to spread the load to multiple threads you could easily make the process slower due to the task scheduler overhead in comparison to just single threaded performance. The performance likely changes with the amount of points your trying to ad...