I watched all the videos. IMO, one thing that would help would be to stiffen up the suspension seat a bit. It appears to be set very soft to help the ride, but what it is actually doing is bouncing and bucking in reaction to every single bump, large or small. Make the seats suspension have to soak up some of the bump before it starts moving/flexing. Make sure your air gauge is working correctly, because to me it also looks like the rears are still over-inflated like they come from the factory for shipping purposes.
Last. Just because it is a stout machine with suspension seat and flex forks, and is supposed to go 15 MPH, doesn't meant that all that will automatically make it ride smoother than everything else out there. Rough ground is rough ground no matter what your one, and even the best suspension system out right now will still buck some.