Far from an expert in Inkscape, but to trace bmp, make sure its selected, then hit the trace function, use edge detection and hit update, preview window should show thumbnail of outline, accept it.

On main window ,slide the bmp which is underneath the outline to the side, delete it, left with a vector outline.

Save as and use Big Blue Saw DXF output , not cutting plotter DXF.

Cheers
Adam