Yes, flecc, your argument makes a lot of sense. In fact, this is the path I am following right now: I am starting to build a couple of e-bikes (which are based on standard 26'' bikes).
If everything goes all right, I will start moving towards my goal, first building say a 20'' mini velo with a foldable handlebar and pedals (but no foldable rear triangle), and not with a frame box, but with a standard main triangle, and a triangular, plastic-like box screwed into it. I believe this can be done in practical terms even by myself.
And so on.
And of course, in every practical build I will discover many new things that now I have not come up with, and even I have not imagined, because only real, practical knowledge leads to real knowledge.
And probably in a few months' time, my ideal bike will be very different from the one I have outlined in this thread.
But I believe it is usually a good idea to start trying things. For example, it would be great to have a collaboration with an engineer who can build CAD models, because once one tries to build that in practice (with real measurements, etc) one realizes of things that before, one had not thought about. And this realization pushes knowledge forward. So, my idea to create CAD models is not so much to have finished models in say two months' time, but just as an exercise to learn. Possibly, the final, "real" model will take two years to finish. In the meantime, of course, I have to build several "intermediate" e-bikes, with as many different motors / controllers / batteries / ... as possible, just to learn.
Having said that, I believe the bike I have described so far makes a lot of sense for European urban usage.