I've been wrestling with this one for years. Nowadays my main mode of travel is small camper van, but I'm also likely to make side trips from it and also to backpack on a long walk, canoe trip or to do day hikes. I also undertake long trips to France and need kit for different seasons. Even in summer it can get down to minus 10 in the Pyrenees. Also we sometimes go in Summer and may not get back till late Autumn. I'm a minimalist when it comes to travel gear but I needed a pack for all eventualites. My current solution is a 50 litre pack, a very light daysack foldable into its own pocket and a response pack rigged as fanny pack or shoulder bag. I tend to think of my kit as a kind of Venn diagram. One set is EDC; the next is the kit I'm always going to need whatever the season or length of trip including day hikes, like FAK, brew kit, water bottle etc. ; the next is sleeping stuff, wash kit food and spare clothing. Anything in the overlapping sets is in the fanny pack. Sleeping stuff etc lives in the pack. Day's food and waterproofs etc in day pack. All of this lives in the pack. On a trek, the fanny pack becomes a chest pack and resulting space in the pack is taken up by food and fuel. Like Emdiesse, I did try a frame with separate bags but I find stuff stuff saks inside a bigger bag works better for me especially when travelling in van car or train. It attracts less attention. The only real problem I find I now have is a tendency to be overequipped on hot days, but I never find myself without anything that matters with a pack of 50 litres weighing 12k. (less food). I don't think there is a simple solution. I've been trying to solve it for 50 years both by experience and avidly reading threads like this. Have fun finding your own solution and share the experience.
The older I get, the better I was.