Wait to usefully import and export STEP you need to be BREP based right? I thought SCAD’s engine was fundamentally incompatible (only really one open source BREP engine out there - OpenCascade)

Pretty much, that's why for DXF export I rolled my own:

https://github.com/WillAdams/gcodepreview/blob/main/dxfexpor...

(shows a DXF w/ arcs)

https://github.com/WillAdams/gcodepreview/blob/main/gcpdxf.p...

(shows a .py file for making DXFs)

See the PDF at the top level for more information.

Yes, AFAIK you are correct that SCAD is incapable of outputting clean STEP files.