AT backpacking cold weather layering question

If you're comfortable hiking in your base layer down to 30 degrees, I think you will be fine for most conditions assuming:

1) You're still going to bring the high loft insulation for your upper body for hanging in camp (I would go heavier than the Cocoon to start)

2) You bring some kind of shell/wind layer for your lower body even though you won't generally be wearing it except in exposed areas and possibly in camp if you don't bring a high loft piece for your lower body

3) You have a good hat and warm gloves plus a light glove shell

4) If the forecast calls for extreme conditions as can sometimes happen, you're willing/able to hang in town if necessary

Edit: I was assuming the Patagonia r2 jacket or the montbell mistral parka was a shell layer. If not, yeah you need a light shell imo. Something like the NF DIAD (9oz for size large) should work. Then I would just bring a light fleece upper body layer to supplement the base layer on really cold days and the high loft layer for rest breaks and camp.

(Disclaimer: I don't have any experience in the southern Appalachians, but I often don't wear much more than a base layer and light shell jacket and pants plus a hat and gloves in the White Mountains in winter. Then I throw on my down parka immediately at rest stops and high loft pants at camp if temps are really really cold.)

I hiked the trail this year and started Feb 15. I too hike really warm and sweaty compared to others, so I know how you feel.
A good WPB jacket with venting options should serve as a wind layer when you need it to. A wind only jacket wont do you much good on the AT and is dead weight. It will probably rain on you alot from the day you start, so make sure to always have a rain jacket.
I actually did a decent amount of hiking in a SS shirt even in the beginning, and used it as a vest of sorts when my baselayer wasnt quite enough, adding the rain jacket after that. Only a few times did I need to also use the thermawrap i carried when hiking. Breaks, lunch, and camp are completely different stories.
I would strongly recommend a puffy jacket instead of a fleece. They are lighter and warmer giving you a much more effective piece of gear, and way better at camp. You would be very thankful for it rolling into a shelter on a cold windy evening after your rain jacket is too soaked to wear for wind protection and warmth. There were a lot of nights like that. Generally the lightest puffy garment will do just fine. many people i hiked with early on had the thermawrap or the 6oz down jacket from montbell, where I had the 14 oz parka.
I hiked almost exclusively in shorts, but carried epic wind pants, rain skirt, lightweight long johns, and puffy pants (till Damascus, you may not need puffy pants starting mid march). If i had it to do over again, I would go with shorts, midweight long johns, and golite reed pants, with the puffy pants till damascus.
A really light wicking sleep shirt may do you right too when all your stuff is soaked at then end of the day

I do a lot of winter and early spring hiking in the Southern Appalachians. For a mid-March start, you'll get weather that varies from very warm days to very cold nights, with everything in between. I've hiked in Georgia in early March in highs pushing 70, and in North Carolina in mid April with lows in the single digits. Like you, I tend to be very warm when I hike.

So, for a long hike, it pays to be somewhat prepared. (Meaning that you can't easily revise your gear load based on the weekend weather forecast.)

I carry very light wool base layers: short and long sleeve tops, and boxer/briefs and long john bottoms. Worn hiking and in camp as a base layer as conditions dictate.

I like the 4-layer system, which is base-layer, wind shirt, insulation, shell. I carry a <3 ounce wind shirt and homemade wind pants, a light down jacket (Patagucci hooded pullover), and a Montbell Peak shell. On the bottom I wear nylon hiking shorts over the appropriate base layer, with the wind pants and a pair of Golite Reed pants as needed.

If I expect very cold weather, I bring Powerstretch tights and a microfleece zip tee for camp. I would very likely carry these items for a March 15 start.

In any case, I bring two pairs of gloves and two hats — one each light and heavy, for hiking and camp. ("Heavy" isn't really heavy, but you get the idea.)

This clothing will handle any weather I've experienced around here, and it's relatively light and versatile.

Have a great hike.

I used almost the exact same setup for a few years and it worked fine. Very versatile. However, in my search for better performance, I shifted things a bit.

The fleece was always too hot to wear when moving, but many times not quite enough for breaks or shelters. The lightweight baselayer was usually good, but I was pushing in in some January conditions (below 30).

I shifted to a long sleeve zip neck Capilene 3 which is versatile for everything I've seen in the smokies, except for when it's too hot and a short sleeve shirt (which I always wear underneath just to have it) works better.

For insulation, I switched to a lighter but warmer down jacket. 3 of them actually, but I only carry one at a time.

Not quite as versatile but it performs better.

YMMV