>IIRC submarine screws use prime-numbered blades (5, 7, 9) to cut down on harmonics. Perhaps the same would be useful here, though I don't see many prime-numbered turboprops,

Might be just odd-numbered ones, not neccesarily prime. But other interesting case is how car tyres have variable feature size on the tyre so the produced sound is more spread over the spectrum and appear quieter

> However my money's on ionic propulsion drones since they have no moving parts to make noise. I wonder if the air accelerated through the grids produces an audible hiss. https://youtu.be/UGM4JXVB5FM?t=126

That would be literally shocking (and changes properties heavily based on humidity) so I'd think use would be limited, it's not exactly great way to lift anything heavy