I will be flying European budget airlines (Ryanair, Wizz Air)
If you have background flying these budget airlines, are the dimensions they provide on their website (42x32x25 cm or 16x13x10 in) absolute measurements? When I am looking for a backpack and the given dimensions are an inch or half an inch larger on the products specifications, can I get away with that by squishing the backpack to meet the dimensions the budget airline requires?

I realize that I am asking for a lot out of this backpack here, but Im hoping that I can find a backpack that can suit my travel as closely as possible

Clothes for 14 days (I can wash them at hostels, but would rather take less time out of my travel to do that)
Small toiletries bag
Full-frame DSLR
One extra DSLR lens, a tripod (might buy a new one thats smaller to accommodate the backpack- so if you have any ideas for travel tripods, let me know), and a few small DSLR accessories (extra battery, charger, two filters)
GoPro and small accessories like chest and head strap, charger, two extra batteries
Backpack that can potentially serve as a daypack and camera bag during my daily tours of cities
Backpack that is preferably less prone to pick-pocketing
If possible at all possible, a backpack that can also hold a 15-inch Macbook Pro and small external hard drive (need to back up media from DSLR and GoPro)
Budget isnt my concern, would rather spend money on a good backpack that will last because I intend to travel a lot of similar trips in the future

I can pack selectively and I am up for the challenge to see how little I can pack, so I was wondering what size backpack you all would recommend for my trip. I will be going to Eastern Europe (Poland, Czech Republic, Ukraine, Hungary) in the middle-late of January, so it will be cold.

I want to save as much money as possible because I am going on 6 flights (not checking in bags and paying extra fees for larger carry-on items for every flight)

Open to any and all ideas! Thank you!

Edit: formatting