I recently used https://convert.emachineshop.com/ to convert a STP file.
From there you could use the open source MeshLab program (https://www.meshlab.net/) to import and reduce the number of polygons.
MeshLab can import the following formats:
Once in Meshlab you can do the following to reduce the number of polygons and simplify the mesh.
I then chose to export a DXF and then import that into Sketchup. In this example I show both a 78% reduction (22% retained) as well as a 98% reduction (2% retained). If materials are involved, you might need to play around with the other export options.
Note, MeshLab does not have an Undo, so you want to make a copy of the original mesh as I do, so you can play with settings until you get the desired results.
I’ve used this technique to reduce poly counts on nice looking assets I find in the 3D warehouse.