Interesting, Vivax went bankrupt during Covid, so you may be able to get one of those at a serious discount.
Batteries aren't that heavy as long as you're willing to accept limited range. A minimal 36V pack (10s1p) with decent Li-ions will do >300W all-day long, if you spec high-current batteries you can easily get 500W sustained. A Luna Wolf Pup pack weighs <4 lbs for a 200Wh pack that can do 1500+ W.
A simple 250W rear hub motor and small pack would be around 10 lbs added to a bike. With some care you could easily get a bike under 30lbs.
The trick is inexpensive. Typically, price is a major driver so most manufacturers cut corners on everything else, so even though the battery and motor only add 10lbs, it's piled on a 30lb bike.