Well, a very light classic steel road bike weighs 27 pounds. Last one I bought was ten bucks. That was years ago, but I see them from time to time in unrestored condition for $50-150. But it's far from small and easy to throw in the truck as a 20" bmx bike. I was thinking more along those lines than folders. I'm not sure what a steel bmx weighs, but I doubt it's more than 30 pounds. A steel 26" mtb might weigh 30-35 pounds.
I get it that if you add ten pounds, doing a clean and jerk lift of the bike to get it aboard is much easier with 30 pounds than it is with 40. So starting with lighter is a benefit. Some folders have very tiny wheels that do make them lighter. I just meant that a bmx might be light enough, and cheap.
Kiss light goodbye when you bolt on a medium strong hubmotor. But there are very light ebikes using a friction drive. The smaller gearmotors are less than the 15 pound weight of typical ebike hubmotors, with tire. I think a 20" bike of any kind, with a low wattage gearmotor should be as light as is practically possible without going to a friction drive on a pricy carbon bike.
I would definitely consider rigging a front bumper mounted hitch reciever, likely a homebrewed thing, then add a swagman bike rack. No need to make it strong enough to tow, just able to support 200 pounds or so. The kind that supports the bike at the wheels. Then you carry the bike up front, and it's quite easy to load up a 100 pound ebike or two.
Or similarly, a bike rack or cargo tray added to the trailer.
Or maybe you just need to rig a step of some kind on the trailer tounge, so you only lift the bike up to the bed from the tounge, rather than trying to go over the truck side.
Another thing possibly worth looking at, are small step stool like devices used by sheetrockers. Looking like a cross between a sawhorse and a bench seat, you'd place the bike on that, then get in the truck, and lift over the side.
I agree, you don't want a 100 pound ebike, but there are many ways to get a 60 pound ebike aboard easy.
