As we are (potentially) driving on all areas and it's steep the membrane is out afaik...Do you have any kind of stabilisation grid with a membrane under the gravel? They don't totally prevent weeds from windblown seeds, but they do keep them from growing up from underneath--in my experience, those are the ones that are problematic.
There is an underconstruction under the gravel to make it a secure driveway... But nothing anti-weed. We've had people come and look and they advised against tearing the whole thing apart, since it was apparently professionally done from a road construction pov