Converting ESRI SceneLayer Package (.slpk) to Wavefront .OBJ, or other common 3D modelling software format (FBX, STL, etc)?

ArcGIS.com has some great Creative Commons data I’d like to work with, but the .slpk format it’s available in is not well supported in the tools I use (QGIS and blender, and I can’t really afford an ArcGIS licence).

How can I convert this ‘ESRI SceneLayer Package’ (.slpk) JSON vertex/material and texture data into a more common format, like Wavefront .OBJ, .STL or .FBX?