Some of the dust I don't think you can do much about. It's dried bits of shed skin and bits from feathers and down. If you are feeding dry feed consider making a damp mash instead. Their scratching can shred the wood flakes and create dust. Maybe switch to wood chips or something that doesn't shred that easily. Part of the dust could be from dried poop so keep the bedding changed out.
I've kept chicks a little younger than yours in temperatures in the mid 20's Fahrenheit, but mine were acclimated. Yours would probably be OK in your temperatures but taking them out there for a few hours for a few days could help. Just keep them out of a direct breeze.
I don't know what you would have in that garage to keep them in. I'd want something big enough that I could heat one small area and let the rest cool off to ambient temperatures. That's what I do in my coop so mine are acclimated. Many people would be surprised how much time they choose to spend in colder spots when heat is available. What do you have to supply heat? I use a heat lamp but heat plates, heating pads, and other things can work. Just make sure they can escape the heat if they want to. You don't want to cook them.