The purpose of the encoder is to allow the control system to determine where the sled physically is (actually, where it thinks it is, based on the number of [partial] revolutions and a knowledge of the screw pitch). That's one way of skinning the cat. The other is to simply think of what your actual use-case is - which is to understand when the sled is fully opened, and when it's fully closed - which the two proximity sensors will allow you to determine.

This simpler solution offers fewer bells and whistles (e.g. stall detection and acceleration/deceleration around limits), but also fewer things to go wrong - potentially higher reliability. You could introduce stall detection with a suitably rated circuit breaker. You'll have to tune the system with the actual position of the sense vane but it should be robust enough for your simple use-case.

I'm not a motors man - a bigger number looks better than a smaller number to me.