Re the Brompton's use of such small wheels, as an owner I would say it copes remarkably well, actually IMO giving a nicer ride than our 20" wheeled Dawes Kingpin. I take it on gravel tracks, but only of the better type and it is "doable".
You need to watch for ruts and big stones and the tyre running pressure being so high is a noticeable riding feature.
I suspect its seemingly long wheel base, that small diameter wheels can offer but are rarely met with just a single hinged folding bike, is a big factor in its ride refinement.
My Brompton, a 6 speeder is not an e-bike, and deliberately so on several counts. Its use is for rides of quite short distances, and those not with killer climbs, but dominating everything is my requirement to have carrying "portability", the whole essence in my case of having a folder. Weight, weight and weight with humping it about, getting on public transport etc. Add to that I did not want the cost of an electrical version, or its drive control system technology. I have a 26" crank drive e-bike for where I "need" an e-bike. I do come to this as a very long term cyclist, now into my eighties.