Make visible only the layers you care for

File -> Save as -> DXF/DWG
Autocad 2007, DXF, Metric
Check "Export pads" "On dedicated hole layers"
Tracks "Export with rounded"
Primitives to export with zero line width "All"
Layers "Currently visible"

Open dxf in inkscape
>Manual scale
>Factor 1.0
x/y axis origin 0,0

Delete via layer

Select the pad which is the origin
Go to transform->move and see where it is in mm by unchecking Relative Move.
Select all objects and relative move -(pad position - pad size / 2) in mm
    Do this for all layers
Select all and relative move -(pad stroke width / 2) IN PIXELS
Delete pad layer

For board shape:
    Select the 2 curves, right click "Move to layer" keepout
    Select all
    Path->Combine
    Go to node tool (below mouse, left toolbar)
    Select all again
    Click "Join selected nodes" in toolbar

Delete layer 0

For tracks:
    Select all
    Stroke style -> Width = the track width (use mm)
    Cap: round
    Path->Combine
    Path->Stroke to path
    
You may need to Path->Union everything for artwork layers to render nicely
    
Extensions->Modify Path->Flatten Beziers (flatness 0.1)
Save as -> Desktop cutting plotter dxf
Use LWPOLYLINE
Base unit mm
Visible layers only

Altium seems to like exporting for these offsets,
sometimes the decimals are different, check them.
-100.579
-112.896
Then do the 0.5px too