I looked at the pics you loaded of the feed bag analysis tag and date code. Nov 2022 is recent, 18% protein is good. The methionine is a little low, so if they were not sick and taking Corid, I'd say give them some vitamins that have B vitamins in them. But for now, I'd leave it until they're better. If the feed looks and smells good, I don't think it's the feed.
I also am not convinced that temperatures are your issue. When I brooded in the house, I would raise up my lamp, and check that right below the lamp was the recommended 95F the first week, 90F the second week, etc decreasing by 5F a week until they reach 80F or the outdoor temperature and can go outside. If right beneath the lamp is the recommended temp and the edges of your brooder are room temp, going by chick behavior is a good thing to do - if they huddle right under for a while, they are too cold, they will ring the lamp at the temp that is most comfortable for them, you just have to make sure they have plenty of room beyond the lamp to do it. For instance, mine would leave about 6-12" under the lamp uncovered, and they'd sleep in a circle about the center. I just had to make sure the walls were far enough away that they could make that "circle" and have room left to expand if needed.
I think you could be fighting egg transmitted diseases, an airborne illness, or genetic issues. You mentioned the first chick was sneezing prior to passing away. There's a number of diseases that will cause chicks to die young or things that can be transmitted through the egg. Might research those if you're interested. The takeaway though is that this may not be something you can help or prevent, and that you should do everything possible not to expose your other chickens or chicks to dander or bodily fluids from these chicks.
I'm so sorry you're going through this. I'm not a long-time expert on chickens, but I've doing this for a year, and have been reading a lot. If I were you, these are the things I would consider.