Don't forget to stir the peaches from time to time whilst baking to make sure the marinade covers all parts of the peach base equally.
I always use ground vanilla, it's my favorite. I never used vanilla essence before (simply because it's not common in Germany), so I can't say anything about using it/ measurements. I'd recommend to just try it and trust your taste buds if you want to use vanilla essence.
I ate the half of my Crumble warm, directly after taking the pictures and the other half later when it was cold. It tasted really good when it had cooled but I would recommend to enjoy the Crumble when still warm.