I backpacked Europe extensively and found that the high cost of goods saw me carrying more household junk around than I would have liked, but I didn't bring a tent.
So in terms of size I think you're looking at something like this
>>2743237.
I valued a clam shell design because it's easier to move the zipper than rummage, and mounting points for a security cable saved my shit twice.
First time someone snatch and grabbed my shit and ended up dragging a table several meters before fleeing, on several occasions someone trying to move it without my knowledge gave themselves away, which might have been an accident or an attempted theft. It made the people who did the wrong thing angry.
Get a pack with a waist band and suffer the awkwardness, or suffer the back injury. Don't discount the value of wheeled luggage, if your 50 liter is long rather than wide you might as well have wheels on the bottom of it. And square packs are incredibly awkward in crowds
