10-day update after the motor upgrade: New motor and chain are working perfectly.

Other news:
As part of trying to further improve the RDF's reliability, I looked into alternative non-contact sensors to detect water level. The most promising are the capacitive units that can detect water right through the side of even opaque containers and pipes. In case anyone's interested, the sensors were:

https://www.ia.omron.com/products/fa...75/lineup.html

http://www.gemssensors.com/level/sin...0-level-switch

https://www.automationdirect.com/adc...ies)/CK1-00-2H

(Thanks to Rayrod2030 for the tip on finding the third unit.)

I ended up going with the CK1 sensor, mostly because of how it can be "taught" what empty and full means, plus its output can pull high or low, something the others can't do.

Once I get past the nerve-racking part of the new pond switch-over I'll try out this unit.