Manufacturers build heavy ebikes. End of story. I think a bike shop could build its own line. If you can get the electronics with the Higo round connectors, it will look quite clean.

My home built e-folder uses a Q100H rear motor on a Downtube Nova bike. Weighs in at 33.4 pounds with a 2 pound battery, but is a bit creaky in the frame. I'm thinking a miniVello with 20" wheels ought to be pretty light with the same drive train.

About 18 mph on the small wheels at 36V and very mild throttle response above 12 mph. I tried 52V and it pedals too easy. I think 48V would be right, but I don't have any small batteries in that voltage.