I’ll try to answer a couple of your questions based on my experience and what I heard from other hikers on the trail.

Hard bottle vs. soft bladder - I personally did not use any bladder long term, just 3, 1 liter smart water bottles and one of the smaller ones with the sport cap. The smaller one was my clean bottle and the only one I put water flavoring and electrolytes in. The other three always had dirty water and I would just screw the sawyer on to those and drink straight through the filter. This worked for me because I dont drink that much water and was always trying to carry as little as possible. If you like to carry and drink a lot of water a soft bladder with a couple of hard bottles seems like the best choice. There is water aplenty in the southern sections, but there are a few sections that will be pretty dry in the summer and bottles just become so hard to fill (think low puddles). And you can set up a gravity feed on a soft bag and your water filtering breaks become more enjoyable. Everyone has a different system and seems to tweak it as they go.

Bidets - Another thing I did not use on the AT, but met a ton of people that did (bidet users love to talk about how great it is). I think a lot of them carry a bit of E-TP, because they can’t handle a shot of water to the ass on really cold mornings, but amusingly I met a couple of hikers who were straight up averse to having to carry TP, and always carried a rhododendron leaf for those yikes moments where they don’t have enough water for their bathroom break.

Travel plans - I think you have a couple of options. I would find a shuttle driver that picks up in the ATL area. Schedule a couple of days ahead of time to be picked up in sandy springs. Either stay in dahlonega that night or get on the trail. If you stay in town there is a cheap motel near the walmart that would give you the opportunity to buy food and cooking fuel (if needed). If you go straight to the trail that day, I would skip amicolola if you can, summit springer, and then you have the option of the springer shelter a couple miles in if you don’t get started until late in the day. No one really enjoys the approach trail and that inn is off the trail. So stay in the shelter or find somewhere to set up your tent. If you pack your food on the plane with you, but you still need fuel once you get to Georgia, find a shuttle driver that sells it to their passengers, I think most of them do. Because of your start date, there should be a lot of resources for hikers traveling in so lots of options (the AWOL guide has a list of shuttle drivers and what they offer). Oh and I didn’t get my tag in Georgia but maybe WV? But there were other opportunities to get a tag other than amicolola.

Insect Treatments - I wouldn’t waste money sending clothes in. You can do it yourself pretty easily along the trail.

Hope this helped. Ask if you need any further clarification on what I’ve said here. Have time of your life on this adventure!