11.4"Lx7"Wx17.5"H(29*18*44.5cm).

have a canon 200D + the 30-80 + 80-300 + macro, charger etc. in my backpack. Am thrilled. The individual compartments can be adjusted very variably. So the 300 should also fit in easily.

YES, I would say so, since the camera compartment as a whole can only be opened from the side facing the back, you cannot see the contents of the backpack, I find that very practical. It is therefore not recognizable as a camera backpack.

Of course, the bag's material is made of water-resistant 900D nylon, it also with a free rain cover

That one doesn’t look like the one I ultimately purchased. I’m sorry but I do not know the answer to that.