Coop Dreams & Chicken Woes: A Beginner's Tale

My best friend and I have started our own coop earlier this year. About a month ago we got about 20 chicks from tractor supply. We had a makeshift brooder setup in the garage. All but one died. We chalked that up to maybe not enough light or heat. She does have an AC leak into her garage so it does stay cooler. So we upgraded. We got some maxicoop incubators and incubated our own eggs. And we ordered and built the 3 layer 15" chick brooder from hatching time. We built it exactly how they said in the video. Our eggs started hatching and transferred about 14 chicks into the top layer of our new brooder. We had ran the heater at least 24 hrs before putting chicks in so any smells were gone. The chicks were in for about a week and then over night last night 9 died. Any help on what were doing wrong? I dont want to cook them so is there a heater temp you find works best for your chicks? These are our 5 survivors 😭