To solve this by CAD, you have to draw a couple of construction lines e.g. through center points or tangents or orthogonal lines to construct the intersection and center points and then use snap modes to draw the entities.
But anyway, your contour will not convert to a polyline because of the elliptic arcs.
A polyline can only contain straight lines and circular arcs.
Also CNC machines don't like ellipses, most if not all don't have commands to interpolate complex curves like ellipse or spline.
Some CAM software has approximation algorithm to convert ellipse or spline into a bunch of tangential connected circular arcs.
In LibreCAD was the option to save the drawing in DXF R12 format, then the ellipse should be converted to a polyline of short straight segments. But as I recognized just now, this doesn't work anymore.
So you have to try to create your shape with circular arcs only.
Maybe there is an online converter for ellipses approximation, but I can't promise that.
Armin
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