Very good plan. I would just write a bit more about the documentation part, which is very important in opens-source projects.
I fully agree with bernd about splitting between GUI and non-GUI components too, and I'll certainly have something to say about the GUI later on

but at this point, I think your proposal is fine and there is no need to have every technical aspect already solved.
One important thing too, I'd suggest you create yourself a github account (or any other git hosting platform like gitlab, but github makes pull requests very easy) and fork the FreeCAD project, and inform your github account in your proposal, so google people can see you have everything ready.
I am not sure you'd actually need to work directly on the FreeCAD source code (your project can very well be a fully independent plugin), but it doesn't harm to get used to git as soon as possible, if you haven't worked with it yet.