The OCCT library can read STEP pretty well, but there's nothing small or light about leveraging that ;-)

F3D rely on OCCT to open STEP and IGES file, just try it :)

The only other Free Software STEP reader of any competence might be the one in gCAD3d. It's written in C. But even if you can read STEP there is the problem of triangulating trimmed NURBS surfaces. I've been wanting to lift that reader and put it in Solvespace for linking parts in assembly but it's a big job and we still can't support all the types of geometry you might find in STEP.

Windows 11 shows STEP thumbnails now.

By default ?