With regard to home building an electric bike that's much lighter than the best off the shelf models, it is clear to me that the only way you'll reduce weight to the levels you want is if you compromise on either features, longevity or safetyI know it's difficult but I want to try.
The stresses due to motor power and propulsion unit weights must be handled by the frame. Additionally the performance characteristics are often higher than unpowered bikes (I'm thinking of the Swizbee website information mentioning superior brakes and suspension were designed in). This means that the overall weight must increase if something else isn't to fail.
By all means try to design a better lighter electric bike but personally if you want the best, unless you're willing to spend a huge amount of time and effort and have the required engineering knowledge, I think your money would be better spent on an off the shelf model - after saving up if needed. You mentioned you enjoyed riding the Flyer T8, the queen of bikes (better than riding a king of bikes I guess
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