Great question! The child should remain buckled until the car has absolutely stopped. You don't want to risk your little one flying into the window if the car behind you doesn't stop, etc. Not worth it. There was an accident in our car line 2 years ago, so that was sobering. 2 cars hit. Please stay safe!
We have the SK Radian carseat, too, for my 5-year-old in kindergarten. He is forward-facing (my 4.5 is still rear in that seat, but she's in preschool). When I stop my car (since to idle is bad for the car and the kids' air they breathe), he unbuckles and hops out. The teacher helper will help him unbuckle. If his coat is heavier, usually he needs a bit of help BUT this seat is pretty easy to undo. Note I never put him in a full winter coat in the carseat, for safety. It's by his feet or over him, and he grabs it on the way out.
We have a van so I open the sliding doors from the front. He used to get out the van trunk (we have 3 carseats in a row in the second row), but then one day the car behind actually hit the teacher. My son could easily be hit that way, so now he is outboard by the curb. I also can reach back to help if needed.
I think you'll find it's not that bad. Congrats on keeping your child harnessed! Mine is starting first grade next week, still in the same carseat harnessed. We love it!
I don't think rushing through is the answer. Once a child gets hurt, it's too late to go back in time. I see kids whose cars are still on the busy road, just sitting on their carseats (unbuckled). I always smile to myself at the parents who take a little longer, and have the child properly restrained. We all want our kids to safely get into the school. (But honestly, your child will get good at this, so she can be properly restrained AND get efficient). Good luck!
The most dangerous part is pulling out of the carline. Some cars want to go around others. It's kind of chaotic. Patience, please. :)