Ah ok yes, second spindle. Good idea. And then have an optical sensor measuring reflectance off the end of it or something like that? What do you think the best way to alter the reflectance is? Black dots around perimeter of white spindle or something? Maybe holes around the perimeter of the spindle?
A couple people on other forums I've been posting in suggested using a rotary encoder to measure the spindle rotations instead of an optical sensor. Any opinion on this?
As for the drone, yes, IS one hell of a drone 
I'm using a modified DJI Matrice 600 Pro with a 6kg payload capacity. May upgrade to a modified DJI Agras MG-1S, which has a 10kg payload capacity. As for line, I'm using high quality kite line to minimize weight, diameter, and stretch.
Motor is still something I'm figuring out. Not sure exactly what type of motor I want to use yet. Suggestions on that would be welcome as well. The payload will be about 1 kg, so the motor would need to put out a decent amount of torque, but it doesn't need to rotate at a high speed. In fact, I'd rather it rotate fairly slowly.
Thanks for your suggestion!