Fold over and isolate the rear delts clean.
Keep your chest down on your thighs and lead with your elbows so the rear delts do the lifting.
Folding your chest onto your thighs locks the torso in place, so the rear delts have to sweep the dumbbells out with zero help from momentum. The bent-over position points the back of the shoulder straight up against gravity, exactly where the raise loads hardest, and the back assists to steady the top of the arc. It is the strictest free-weight rear delt move you can do with nothing but a bench and light dumbbells.
The seat and the chest-on-thighs position remove the temptation to bounce out of the hips. Standing bent-over raises invite a heave on every rep, seated you either lift it with the rear delts or you do not lift it at all.
You are lifting too high or shrugging as the arms come up. Stop the raise at shoulder line, keep the neck long, and lead the movement with your elbows so the back of the shoulder does the work.
Embarrassingly light is correct. The rear delts are small and the strict position gives them no help, so pick dumbbells you can pause at the top with. If the weights need a swing, they are too heavy.
