I'm treating myself to a 2 month road trip as a retirement gift early next year. Since I will be travelling by vehicle room for "stuff" isn't a big issue. Accomodations will be a combination of staying with family, motels and a bit of tenting. I'll be watching the advice given here for my planning!
I love my Cotton Carrier for birding. My usual birding set up is a Nikon D500 with a 300mm prime and 1.4x converter. I have not tried it with an 80-400 lens. Very comfortable to wear and it will keep your camera and lens secure and not swinging around as you hike.
I have a Mountainsmith Tour lumbar pack. I find it extremely comfortable to wear all day. It is quite roomy. I do find that if I use small zippered bags to organize my stuff inside it's more functional. I have a couple of zippered pencil cases I got from LL Bean that work well and are fairly rugged. Batteries in one, memory cards in another and any allen wrenches needed for tripods/heads, etc in another. Still room for my phone, snacks and whatnot in the pack. Water bottle(s) in the mesh pockets on the sides.