I've got one for my motorcycles (although mine is all steel, much heavier duty than you need for any bicycle). You see hundreds of them at dirt bike events. To me the license plate thing is a non-issue.
I have one that I used to use when I had a dirt bike, and I have used it for bikes a few times. I attached to the seat rails and the handlebars and strapped the front wheel so that it wouldn't flop around. It worked fine.
What are you talking about? There are exactly zero issues using tie-down straps on carbon bars. You're trusting them to hang together when your fork bottoms out on a 40 MPH rock strewn downhill but you don't think they can handle the load on a gently suspended vehicle? Um, no. They'll be fine.
I like the OneUp racks, but they're 2-3 times the cost of a cheap dirt bike hitch rack. A beautiful piece of engineering and machining (we have one and it works very well), but not necessary, and if you also need to haul a dirt bike, obviously not an option.
As a serious dirtbike rider, my question is why are you considering a dirtbike carrier for ebikes? I can carry 2 bikes on the back of my Volvo wagon on a Yakima bike rack that fits in my trailer hitch.
I had some steel welded to the main stress point on my normal 1 1/4" hitch rack mount. It's original weight rating was 100lbs. My two emtb's without batteries are less than 100lbs so the extra steel and removing the batteries provides a large safety margin. Find a local welder and he should be able to beef up your hitch mount rack for a minimal charge.