Over time I've whittled down weight and volume quite a bit. I plan to ride 2+ months this summer with a base volume of 21L; base weight in the neighbourhood of 10Kg. I use 2 x 5.5L Ortlieb fork packs, one for clothing, the other for hardware (actually partly mixed to get to even weights) + 2 x 5L dry bags lashed to the read rack, one for the tent + pad and the other for sleeping bag + liner. I also carry a 25L (MLD Core) backpack lashed to the rack, for food items.

My suggestions would be to:
(1) Limit weight and volume as much as possible -- there comes a point where you no longer have to walk stairs twice, once for the luggage and then for the bike, which is liberating.
(2) There are stores everywhere, so it is very unlikely that you'll die from hypothermia because you didn't bring enough layers
(3) Putting more load on the fork makes for a more stable ride, especially when riding uphill.

(4) I started with the typical 2+2 Ortlieb kit, which gives you 65L of carrying capacity. "Graduated" to front panniers only (25L + tent and food on the rack). And it will be my 3rd year with the 4 x 5L kit. Less is more