When it comes to shoulder health and fixing issues like excessively rounded shoulders you have to spend time strengthening the shoulder as a whole. You have a lot going on in and around the shoulder with more than one muscle making things happen.
This is why I always suggest strengthening the shoulder through multiple ranges of motion at different angles.
The rear delt complex is an exercise you can use in a rehab setting or general strength and muscle building situation.
Pick a light band and see how well you can maintain a stable shoulder and trunk while working through each rep!