How about a totally different design?
Seeing the sprockets that put tension on the extra chain made me think of this design idea. Instead of a fixed ring, use 2 chains (of equal length) wrapped around a circular piece of plywood (or something) attached to the sled. The circular plywood would need to be precisely centered with the router bit, and it would have a double groove around the outside to retain the chains. The loops of chain might be secured with some clips or something, and the chain from the motor would be attached to an idler sprocket.
I haven’t built a prototype yet, but think this would be super simple and fairly inexpensive. The only parts are some plywood (which might just be lying around anyway) a couple idler sprocket pulleys, and some extra chain. The size of the sprockets, and lengths of the loops might have some effect on the performance, I still need to think about that a bit.
It could probably be done with some paracord and v-groove pulleys, but it might be difficult to get the loops exactly the same length.
Forgive my sloppy inkscaping - first time for everything 
