I have gone through the same process of looking for rendering software that will accept non-mesh style formats like STP. I don't like the hassle of exporting to a different format and uploading to the cloud for file storage, but Fusion 360 can be acquired for free if a user earns less that $100k using it commercially. It has a decent enough render engine and a user can get something right out of the box, so to speak.

Here are some models I rendered with Fusion 360: http://www.textualcreations.ca/Flash_Gallery_Fusion/Fusion_Gallery.html
Not many but I don't have near enough time to play and experiment.

I believe I learned about the existence of Fusion 360 right here in the MoI forum.

I have been looking at SimLab but I think my trial is going to run out before I have enough time to 'render' an opinion.:-)