People assume a longer trip needs a bigger bag. The opposite is true. The longer you travel, the more a heavy case punishes you, and the more you notice how little you actually wear. For a month, or even three, we each take one carry-on and a small daypack.
The trick is to pack for a week and plan to do laundry. Choose one colour palette so everything goes together, take layers instead of bulky coats, and bring shoes you can already walk all day in. Most swap homes and apartments have a washing machine, and a small bottle of travel detergent covers the gaps.
Leave room for the things that matter more as we get older. The medicines, with a printed copy of the prescriptions. The good reading glasses, plus a spare pair. Chargers and a universal adapter. A folder, paper or on your phone, with copies of passports, insurance, and bookings. That folder has rescued us more than once.
Everything else you can buy there, and buying it there is half the pleasure. A scarf from a market, a jar of honey, a bottle of the local wine to leave for your hosts. Pack light enough that you have room to bring a little of the place home.