Just over a year ago my bag fetishist husband gave me a backpack as a gift (my father had passed away and he wanted me to have something special).
It means I’ve had quite a few trips with it. Had to be comfortable, ergonomic (back problems mean that constant postural hygiene is essential), easy to pack and retrieve from, enough structure to stay organised and to keep electronic screens protected, sized to comply with all but the smallest cabin luggage limitations, expandable for other times, and looks smart enough not to provide embarrassment when used to arrive at formal events.
It also has an unusual downward facing “external” flap with magnetic fastenings. You can put something sodden wet there (protective bag covering, raincoat) and it won’t have any impact on the rest of the contents. Bag comes with straps and tethering loops for extras (including secure attachment to a roll on case).

It’s the Peak Design Travel Backpack (mine’s in sage). Zipped up for air travel its 35L capacity. Unzipped and expanded it’s 45L.
Essential for me was the additional purchase of a hip belt which securely integrates itself with flaps that are quick and easy to hide within the bag’s external architecture.

I also got a medium sized compression cube from the same company. It’s awesome, best I have.

Was as handy for an overnight stay on Mount Fuji (getting yourself and a child organised at 4am in a cramped, dark environment surrounded by people sleeping was a formidable test) as it was for a “Sunday Best” family gathering at a hotel’s restaurant. I had expected the Fuji San expedition to age and wear the backpack (two days of dust and abrasive gravel) but it just shrugged off the experience, no signs it ever happened.