II can’t answer your question. I have no idea how cold it actually gets in there, what wattage bulb you are using, or how draft free it actually is, thought I suspect very draft free as long as the garage door is closed.
Have you considered reaching in here, picking up the thermometer and looking at it? The chicks really won’t mind that much. I agree with Azygous that the behavior of your chicks is most important, but since this seems to be your first time, you can get a lot of self-confidence with that thermometer. Once you’ve been through it once you will understand much better about their behavior.
The basic set-up looks pretty good to me. They should be able to get away from the lamp if it is too hot in there. That’s important. Since that is so well ventilated (which I like) they will probably spend a lot of time under the heat, but they should roam around some even if it is pretty cold in there. The older they get the more they will roam in the cold. Do you happen to know how cold it gets in that garage at the coldest? If it is an attached garage and you don’t open the main garage door it’s probably reasonably warm. Some of us brood in coops where it gets well below freezing.
Usually by two weeks they can fly really well. But since you have wire sides and they do not have a solid place to land so they will probably stay in there fairly well. You might consider putting some kind of top on there, netting or something, but remember to not start a fire with that heat lamp. Chicken wire might be best.