100 Reps, Unbroken If You Can
Add this somewhere near the end of your delt workout and aim for 100 reps. Try to get all of them unbroken, but if you need to pause and take a few breaths, that's fine. Just keep the breaks under 20 seconds.
Start the movement with scapular PROtraction. Your shoulders should be rolled forwards and not pulled back when you initiate each rep. When you initiate with scapular REtraction, the mid-back will hog all the tension and the rear delts will get far less.
So don't initiate the movement by pulling your shoulder blades back. Keep them pushed forwards almost like you're hump-backed. If you are naturally hump backed, I apologize for making fun of your inability to initiate with scapular retraction.
If you don't have access to a band, the rear delt machine will work. Just pick a weight that lets you hit the full 100 reps with as few breaks as possible. Your rear delts will thank you and your overall shoulder health and posture will improve.