Make visible only the layers you care for
Select all art:
(Layer = 'TopOverlay') And Not IsComponentPrimitive
Then move to "Artwork Export" layer

File -> Export -> 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

Select the pad which is the origin
Go to transform->move and see where it is in mm by unchecking Relative Move.
Offset is pad position + (pad size / 2) + (pad stroke width / 2) (in mm, often 0.1325mm)
Reopen file and use new x/y axis origin

Delete via layer
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 artwork laser cutting output:
    Cross reference with altium to make layers for tracks
    Use filtering, it's fast
        1.5mm
        1.2mm
        1mm
        0.75mm
        0.5mm
        0.254mm
        Raster Fill

For tracks:
    Select all
    Stroke style -> Width = the track width (use mm)
    Cap: round
    Path->Combine
    Path->Stroke to path

For raster:
    Path->Combine each shape
    Go to node tool (below mouse, left toolbar)
    Select each shape one-by-one, select all nodes
    Click "Join selected nodes" in toolbar

Move everything into 1 layer
Path->Union everything for artwork to render properly
    
Extensions->Modify Path->Flatten Beziers (flatness 0.1)
Save as -> Desktop cutting plotter dxf
Use LWPOLYLINE
Base unit mm
Visible layers only