I have read a number of threads related to shrouded propellers. And, if the primary goal were to significantly improve the safety of a drone used for close inspection of buildings and roofs, then any net efficiency gain would simply be a bonus. Based on what I read though, the net efficiency of producing static thrust may be as much 50% to 60% even after deducting for the additional weight of the shroud. This would allow for either of three different design changes. 1) a smaller overall size and mass resulting is reduced risk (increased safety) 2) longer flight times for the same size and weight of the drone and payload 3) greater payload capacity for the same size and flight duration.
In the case of a drone used for inspection there is no compelling reason for flight at high velocity, or great distance, or great height, and the design criteria become more a question of safety and survivability, flight maneuverability and stability, flight duration, and a manageable drone size for transport. Most inspectors are not hiking into the woods or countryside to fly their drones. So the ideal size for transport is simple any size small enough to fit into a modern private vehicle. So an overall size of 20" to 30" is not too much different that what is currently being marketed for the majority of drone pilots. But, it could still be argued that a smaller hovering platform is a better match for the needs of more inspectors.
With all this said, are there any shrouded propellers being offered for sale that are in the range of 6" to 8" in diameter (150mm to 200mm)?
Please note: There are threads on the topic of shrouded propellers that include comments and responses about ducted fans with links to products that are designed for use in model jets, and VTOL fixed wing craft and they are not well suited for producing static thrust due to overheating. And there are still other research papers that discuss uses for shrouded propeller as the primary powerplant on fixed wing airplanes. Too many of the design criteria for those applications are not relevant to this discussion.
And, with a multi-rotor drone that would use shrouded propellers, the need for complex steering mechanisms is presumably non-existent for the same reasons that all multi-rotor drone do not need them. So, many of the research papers that are available are simply not relevant to this discussion.