Does ducting help to stabilise heavy lift drones?
There is too little information given to make a definite answer here, but things you should take into consideration are:


depending on the construction the bigger crosssection if keeping a similar rotorsize which will likely increase drag at higher windspeeds while on the other hand leading the airflow so with the right construction it might behave better then without ducting.
using at least 6 or better 8 rotors should enable you to increase lifting power and stability
putting the rotors in an angled position should increase stability while it might slightly reduce lifting power


additionally


the ducting will offer protection for the rotors/fans and people/things getting hit by the drone in case of a crash
as you want too operate it for such a long time the increase in weight which might increase the number of refuels/battery changes needed


Also we were talking about drones that produce the whole lift by rotors pushing air downwards. What about using a ballon or some kind of plane? This will make it more sensitive towards wind and sacrifice maneuverability but will allow extended flight time and potentially higher efficiency.