Similar to associating DWG file to launch AutoCAD:
https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/DWG-file-type-asso...
But instead associate DXF file to AutoCAD Launcher from Windows Explorer File Properties
Notes:If DWG launcher utility is associated with Dxf files, the last-used AutoCAD program is launched (assuming multiple AutoCAD programs are installed) when double-clicking on a Dxf file. The AutoCAD DWG Launcher application manages this process. Dxf files are not associated with acad.exe but rather with AcLauncher.exe.The DWG Launcher application is located in C:\Program Files\Common Files\Autodesk Shared\AcShellEx\
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