I've downsized over the years from Osprey Porter 46L to a Farpoint 40, and now a Tom Bihn Aeronaut 30L travel pack. I spent 3 months end of summer/autumn (Aug - Oct 16) in the UK and Ireland last year with the 30L pack, and it weighed in at under 6kg - here's what I packed
2 x Outlier long pants (wore one on plane)
2 x Outlier shorts
1 x Prana shorts
1 x Quicksilver boardies
2 x Ice Breaker Ts 100% merino
2 x Smart Wool Ts 100% merino (wore one on plane)
2 x Ice Breaker long sleeve Ts 100% merino
1 x Outlier long sleeve collared shirt merino/synthetic
1 x cashmere V neck jumper
1 x Patagonia R3 Fleece reversible Hoody (wore on plane)
1 x Arc'teryx Cerium SL Down Vest
1 x Arc'teryx Atom Lt insulated jacked (carried)
1 x Outdoor Research Helium Rain shell
1 x Outdoor Research gloves (didn't wear them)
2 x Ice Breaker Boxers 100% merino (wore one on plane)
2 x Ice Breaker Socks merino/synthetic (wore one on plane)
1 x pair of comfortable black leather boots (wore on plane)
1 x pair of Keen Clearwater sandals
Carry on toiletries bag (face wash, toothpaste/brush, floss, mirror, roll on deodorant, Sea to Sumit super concentrated clothes washing liquid)
Close line - surgical rubber band (no pegs needed)
Chamois Towel
Sea to Sumit Dry Bag (improvised washing machine / dirty laundry bag)
Glasses - Reading/Sun / iPhone 6+/ charge cable/ plug / UK adaptor / Recharge battery - iPhone.
With the exception of the underwear and socks everything else can be worn multiple times before it needs washing - 100% merino does not hold body odors. And when you do wash it, it drys super quick. I use the Dry Bag as a mashing machine either in the shower or the sink - put your clothes in a few drops of the super concentrate, you can even use shampoo, fill it with water then either close it up and shake or use your arm to agitate it around, rinse it out, wring it out, roll the clothes up in the hotel towel and hang, most times everything is dry in an hour or so. I then use the chamois towel to dry myself. Chamois towels are great rinse them out in hot water and they are good to go. Those travel 'PAC towels' you see advertised are terrible, they stink to high heavens after about a week even after washing them in a washing machine.
Cotton is another thing to avoid if possible, the only thing good about it when traveling is that it's cheap. It takes forever to dry and you can only wear cotton once, then it needs to be washed. Merino is great - but it's expensive, there are a lot of good synthetics out there today, which are ok.
I stayed at over 20 different destinations/hotels on my last trip, and the only documentation/paperwork I carried was my passport, international drivers license, copy of my travel insurance and a one Page itinerary with address/email/phone of where I was staying and transport details - hire cars/trains/planes/ferry . I scanned (PDF) all of my other documentation and emailed it to myself and also used an app on my phone to save the PDF on my phone.
Anyway - good luck with the 20L endeavor :-)