I like Build123d but I really want a hybrid mouse/code CAD built around it. I want to be able to click on entities and have them show up in the code editor instead of blindly trying to select edges.

Agree 100%.

I've tried various code CADs before, and they're fine as long as what you're creating is simple enough to build from independent pieces, ideally that you can keep in your head. As soon as you need to refer to aspects that have been created within the CAD, like an edge, or a particular face of an extruded shape, the paradigm falls apart.

This feature technically already exists in build123d + OCP CAD Viewer (ocp_vscode). You can click a feature in GUI and it will copy the index to clipboard, you can then apply e.g. fillets to it on the code side. See here https://github.com/bernhard-42/vscode-ocp-cad-viewer/blob/ma...

The biggest caveat is that there is currently absolutely zero mitigation for toponaming. This feature is extremely brittle, so I tend not to use it very much -- but I am still glad it exists for those situations when composing a selector is too annoying.