There are a few things to consider. What does the coop look like you are putting them in? If it has good draft protection where they are sleeping and decent ventilation, you are a long way toward being OK. The way I accomplish good draft protection with good ventilation is to have the ventilation openings over their head when they are sleeping.
Another important consideration is age. Yours should be fully feathered out by the time they are 5 weeks old. Once they are fully feathered, most of your worries are past.
How warm is your garage without the heat lamp? How is your brooder set up? I keep my brooder in the coop from Day 1. My brooder is fairly large and well ventilated but I can provide good draft protection if I want to. In the heat of the summer I don't but in cold weather I do. I heat one end and let the rest cool off as it will. Sometimes with very young chicks that gets down in the 40's Fahrenheit at night. I would not worry if it got even colder as long as I have the heat at one end they can go to. Many people on this forum would be shocked to see how much time they choose to spend in the cooler parts of that brooder, just going back to the heat to warm up when they need to.
I do think mine get better acclimated with the time they spend in the cooler parts. Last fall, I took the chicks out of the heated brooder and put them in an unheated grow-out coop when they were 5 weeks old and overnight temperatures were in the 40's. When those chicks were 5-1/2 weeks old, the overnight temperature hit the mid-20's Fahrenheit. Those chicks were fine.
When you put your chicks out, they will probably not roost at night but will sleep huddled down in the bedding in a pile. They do that in the heat of the summer too. It’s not to keep warm, they just like the company. With yours, the added benefit will be that they do stay warmer, both from the insulation from the bedding and their body heat.
I’d leave them locked in the coop for a week or so anyway when you first put them out. It helps tell them that the coop is home but also keeps them out of the worst of the weather until they get a bit acclimated.
I don’t know if your garage is attached or detached or how cold it gets in there. Don’t be afraid to give them some time in colder weather in there if you can. Turn that heat lamp off some, especially during the day, to help them get used to cooler weather. If they get cold, they'll let you know by their distress peeping. When you hear distress peeping, you recognize it. Before you put them in the coop, you should leave them without heat in the brooder for a few nights, even if it gets cold in there. And definitely let them go through a few days without heat.