I can't see needing a bivy sock on the AT with a March 1 start. Might be really nice on a couple of nights, but not worth carrying IMO. Especially if you end-up with the door option on your tarp.
A full sleeping bag is a bunch of extra weight you won't want. If you can get a quilt prior to your start, you'll be happier for it. A 3-season will get you thru with a little extra clothing at the start.
Best of luck! FYI, I have a doormat with your username on it.