Onshape is a decent middle ground here. The BREP kernel (parasolid) is obviously closed source, but the standard features built on it (extrude, loft, etc.) are open-source and written in their own bog-standard c-family language of FeatureScript[0]. You can write your own custom features pretty easily, and the feature inputs and outputs are actual BREP. It's easy to mix and match manual modeling with standard features and automated modeling with custom features.
[1] https://cad.onshape.com/FsDoc/