Someone simply needs to invent a motor that provides constant torque through a full revolution.

To control the amount of thrust changeable rpm is needed, or a changeable geometry, like pitch. I imagine the latter wouldn't be easy with the toroidal propellers.

You could mount each toroidal blade on an angled pivot and use rpm control to change the pitch. This enables thrust vectoring.

This might be what you're referring to, but Tom Stanton built a swashplateless 4 axis R/C helicopter (albeit not with toroidal blades) based on this principle: https://youtu.be/d80oXSCcHTk