Well, the backpack definitely makes a difference, and every little bit of space helps. I was using IKEA’s 27L foldable backpack. (This was the one, but it seems like they've retired the product in the past few years since I last bought one: https://de.ikea-club.org/item/90330483.html) Since it was flimsy, it had a little bit of stretch. But to this day, I still haven’t seen any other company selling a collapsible backpack of comparable size.

In any event, I took my extended trip with this backpack in the spring/summer months, when clothing is light and small. But if you doubt that you can fit this all, try it! You’ll be VERY pleasantly surprised. :)