Delt “sweeps” work well for high reps in metabolic stress fashion:
The execution is really important. On the lifting side, it’s really a pull back – as if you were doing a face pull and lateral raise hybrid. With the supporting arm you’re going to be pushing slightly as you lift with the other arm. (It’s not a one-arm upright row motion.)
If you get this right you’ll feel it big time in the lateral delt and even some posterior delt and traps. Start light and play around with it until you can feel the contraction.