I have two electric bikes—a touring bike, which I use primarily on the road, and a mountain bike which I use primarily off-road. I really DON’T want to record both rides under eBike as they are very different rides.
Moreover, when I choose "Mountain Bike," Strava ASSUMES that it’s not an electric bike. This was made clear to me when I got a message from Strava that I was number 2 on a particular trail. That probably would not have been the case if I’d been on a non-ebike!
So, my suggestion is that you have another category for electric mountain-bikes, ie. eMTB!