Tip velocity leads to infra sound, audible noise and strike risk. A solution to this would be smaller, multibladed propellers that create higher torque at much lower tip velocity.

Technically a wing will produce force at zero RPM and there would not be an efficiency loss as such. The settling on 3 bladed propellers is largely due to their mass (hence why to resolve this they need to be smaller). Making them smaller reduces power, unless you combine them with cowlings.