I have a fixture, but I think people that don't have one might consider following Paul Brodie's minimalist approach. Although he does tend to run to the lathe or mill when he needs something. https://www.youtube.com/c/paulbrodie/videos
I have been thinking about doing something similar. Sorry I don't have any answers for you at this time.
I have a fairly extensive collection of industrial motors. I hadn't thought about using a gearbox with them, that's a good idea. Too bad motors like that don't usually come with a right...
It's really disappointing they went with a proprietary crank. Probably saves them money somehow. And OEMs and most of their customers most likely are not going to care. I'm sure OEM sales are much more common.
I don't think it's guaranteed that the bb insert will loosen up. They are in there pretty securely. Carbon will cause corrosion in aluminum if the builder isn't careful, and that would cause a loose insert. Like I said on bikeforums, I'm more worried about the battery.