Would this be a worthwhile addition for rotator cuff health?
https://www.amazon.com/ShoulderHorn-Medium-Rotator-Training-Device/dp/B0042JHTKE/ref=sr_1_47?keywords=rotator+cuff+exercise+equipment&sr=8-47



Or would grabbing a light dumbbell and doing this type of rotator cuff exercise be fine?

The rotator cuff muscles are stabilizing muscles and they should be strengthened in various ranges, planes, and movements. There's no need for a devices that's going to put your shoulder in a specific position to strengthen.

The seated shoulder external rotation is a great exercise, but I'd find a few others to put in your routine as well.