Hilltoppers are certainly not hard to install, but starting at $799 with only a throttle and you need a bike, prove my premise, which is that I cannot convert an ebike for less than what it costs to buy the least expensive ones. I don't think hilltoppers come in 20" wheel anyway.
Sondors Step is one bike new to me. I didn't realize it folded, though I can't see much advantage if it has 27.65" tires.
I've seen a Sondors Fold-X down near the St Peterburg waterfront chained up to a parking meter. It occurred to me that it could have been easily lifted up and tossed in a thief's car. It's a nice looking bike, but I would get rid of the noisy tires and put on smaller 20x3's.
Another folder that is truly intended for a 4 mile ride, and carrying on a train, is the velomini at $1200. It's supposed to be 31 pounds.
https://velomini.com/buy/velomini-plus
For me, it's no good because when I ride a bike, I want an enjoyable ride, and with the little wheels, that's a bike where you're watching for bumps and potholes more than usual.