Blender

Try to unwrap uv, may work

Hello Adam

You may apply a loop cut, then select the area or line that you want to flatten and then scale it to zero in particular direction....Then you can get flat surface...

I hope an example can solve your doubt...

Regards

Mayank Shakya

My tip is that: is the use of UV-Map unwrap the faces you need to be flatten in UVEditor and export this UV as an SVG and clean it inside, for the scaling it in the correct dimensions, the correct way add a reference (a flatten rectangle with dimensions that you know, eg. 2 meters ) inside your mesh before unwrap and export the result into SGV then open with inkskape, and export as DXF and edit the scale in LibreCAD or QCAD or with your preferred one, you can also re-scale it, with Blender by importing SVG or inside inkscape.

this is the layout with rectangle with of 2meters

and here I've imported the svg and conveted to mesh and joined and added a measurement

then I scaled it by the result of the get from the python console with a simple divisions and pasted the result into scale object, the image also show a flat object only with the boundary edges, to obtain this you can use the decimation modifier with planar option and angle to 0.1

and here the same object in QCAD, I had a problem on exporting to DXF and I've converted into again into curve after cleaning and imported as SVG but as already said you can use inkscape that has the feature to export in DXF R14

Happy Blending.