Agreed: my mom's e-bike is a bit of a pig to ride, because it's got a lead acid pack and despite an aluminum frame the whole thing is heavy. 65 lbs?
But if you truly only want occasional ASSIST, you can actually build a lightweight electric hybrid. How does a 100 watt motor and 4.2 pounds total system weight sound?
See:
Super Lightweight Assist -An ultra light-weight DIY electric assist system for a bike or human-powered vehicle
The problem with most e-bikes & kits is they're designed to be e-bikes you
sometimes pedal.