One of the tips that I found from the Clean My Space YouTube channel regarding oven racks was to line your bathtub with a large towel and then place you oven racks in hot water to which you add powdered dishwasher detergent and allow them to soak overnight.
I was unsure of how well this would work, but was amazed at how clean the racks became. I only needed to take a Brillo pad and scrub just a few little areas that needed a little more elbow grease.
It was almost magical how well the detergent removed so much of the stains without having to scrub llike crazy.