I've done my fair share of bent-over lateral raises (rear delt raises), but the truth is that they fall short.

With a bent-over lateral raise, the rear delts never actually get fully shortened. A key component in maximizing an exercise is to get the target muscle fully lengthened and then fully shortened within a movement. But some exercises don't do that as well as others.

Here's a better option:

Rear Delt Row with Supination

Row to bring the elbow behind the body as far as possible, which will shorten the rear delt. Add a twist, literally, by supinating as you perform the row.

Why the twist? Because it'll bring about a certain amount of external rotation in the shoulder, which is one of the components of the rear delt. Supination at the forearm is actively linked with external rotation of the shoulder, just like pronation is linked with internal rotation.

This isn't an exercise you'll go super heavy on, but you won't need to so long as you're using the appropriate resistance with it for the mechanics, and making sure to get the rear delt fully shortened.