You have something very wonky going on with the units in your SVG file. It’s simply a corrupted file.
Any time I try to change the display units, it keeps reverting back to pixels. Normally in Inkscape, say if you set display units to inches, you will see the numbers in the SCALE box automatically update to 25.400 because by default Inkscape likes to work in mm. This file however does not update the scale like that, and changing the units from pixels is futile. The changes just won’t stick like they should.
However I did get it to import into the GFUI at proper size after saving as Plain SVG file format. The screenshot below shows the imported file in the GFUI and on the right you can see the Inkscape Document Properties in the window behind the GFUI.
The fix for your issues is simply File>New and stop using “Save As” to create new files.
Here’s a bullet point workflow for importing Fusion DXF’s:
In Fusion:
1: Set Units to mm or in. Doesnt matter which, Inkscape plays nice with either.
2:Export DXF.
In Inkscape:
3: File>New
4: Document Properties> Display Units set to mm or in. You could use pixels but it’s a useless unit in the physical world.
5: Document Properties> Custom Page Size: Set units to in then set Width to 20" and Height to 12". This helps the GFUI read the scale properly if it can’t use the units embedded in the SVG file.
6:Import the DXF file and when the dialogue box comes up asking for Scale Factor, select the “Read From File” option. If the DXF is in Inches and the Inkscape file is in millimeters, or vice versa, Inkscape is going to scale it properly.
I’ve never had a scaling issue with this workflow.