I didn't know OnShape had such a feature. Will check it out!
What you describe is one of the main reasons why I use Rhino3D. It can be scripted via the Grasshopper plugin, which integrates really nicely with Rhino and its primitives. Sadly, Rhino isn't open source and is quite pricy
The fun thing is that onshape itself has a very thin kernel. Most of what you see as built in features are actually featurescript based. Onshape provides the source code for their built in feature set as a reference. https://cad.onshape.com/documents/12312312345abcabcabcdeff/w... You do need an account login ( free ) to view it.