The DXF-File is a bit crappy. Wholes are not exact circular somehow squares with rounded edges.
rounded edges are build from several secants instead of a part of a circle.
Why the heck did you make it so complicated??
If I want to use a CAD-CAM-Software (like ESTLCAM) to create milling GCode it does not work exactly becauses circles arent exact circles etc. Don't give OpenSource a poor quality image!
@andrewKI you can conclude thickness through measuring parts where the main-plane fit in an angle of 90 degrees and after fitting both parts build a straigt line. 3mm I think.