zzzquil is just benadryl. It takes a lot to truly "pass out"
Alcohol does amplify the effect of it though, so with enough of both...
but really if you're willing to try that method one thing described in the final exit and pph is bag over the head with elastic band, holding open so fresh oxygen flow with your thumbs (in seated position) and after a lot of sleeping pills you will fall asleep/pass out, your hands will drop, the band will contract and oxygen flow stops... there is risks though, your unconcious SI can still be enough to tear off the bag, you might not drop both fingers etc etc
there's risks with those methods for sure