First, get a backpack that holds a lot. Something like this:
https://www.amazon.com/LOVEVOOK-Backpack-Waterproof-Weekender-Overnight. There are tons of options so focus on what you need from that backpack - water bottle holders, laptop sleeve, organizers, shoe bag, clothing, etc. They fit under economy seats.
I was able to travel to Japan for a week for business and go directly to a long weekend in Vegas (so very different clothes) using a backpack like that and a standard carry-on rollaboard. Granted, it was during June, so the clothes were lighter weight than winter. I wore layered lounge clothes on the flights (the same outfit to/from Japan) with sneakers. Had one pair dress shoes and one pair sandals packed. Streamlined my toiletries. Did not pack hair dryer or curling iron bc voltages are different. Packed lots of underwear and tank tops bc of the humidity in Japan. Never washed anything.
Good luck! Planning ahead and editing works well.