I'm not a fan of isolation exercises in general, but since it is your question, I would suggest putting yourself in a bent-over row position and perform reverse flys with dumbbells in a pronation grip

Lower weight and higher rep is better since the rear delt is a small muscle and using too much weight will result in the activation of other muscles to compensate. 10-20lbs should be enough for this.

This is a picture of the exercise: https://i.ytimg.com/vi/Tm_6m8iaVrM/maxresdefault.jpg

Notice how his hands are sideways in pronation grip. Not sure why he's resting his head on a bench, that is un-necessary.

I don't know if you've tried this yet but, how about doing cable rows a bit higher to get more rear delt and trap activation? It helped me more with traps then rear delts but I still saw a difference after a few weeks.

Tenshans

Bent over dumbbell lateral raise. It’s the bomb.

Are you doing anything other than facepulls?

Incline delt row either straight up and down with a barbell or fly motion with dumbbells