"It sounds robotic even on slow speed" Solution: Check your scale. If the singer is slightly off the intended note, force them to sing in tune first. No plugin fixes bad source material.
"Formant shifting sounds unnatural" Solution: Reduce formant correction to 50-70%. Also check that you're not pitch shifting more than 2-3 semitones.
Allows you to select specific musical scales or program custom note grids.