hi again, first of all, so you have a 20 litre backpack on? I havent ridden with a backpack or knapsack on for over 30 years, so I dont really remember how it was, but I do know I never liked it and when I got my first bike with a rack, I have never ever looked back. I realize that bigger rear panniers and/or front ones will not change the weight, but I would strongly recommend ditching the backpack. Sweaty, harder on your back, maybe even neck and shoulders. One would think that it is working against you. Your choice but I sure wouldnt go with it.

as for your fitness, doing that number of klicks per day must mean you are in pretty good shape leg wise and otherwise. In reading your oringinal post, it was hard to know what sort of shape you are in, hence the comments by myself and others. I dont mind climbing, perhaps cuz I am light, but again, its pretty much impossible to get an idea of how someone rides by a forum thread.

My touring average speed was pretty consistant over the years, somewhere around 16-17 kph ( a smidge better on good roads, only flat stuff, tailwinds etc) so not much different than you. Another factor or perhaps diff, I was on a 700 wheeled, 28 tires at 105psi all the time, which roll faster by nature than 26 wheels, even iwth slicks. I have commuted for years on a mtn bike with 80 psi slicks, and my touring bike is a number of kph faster with the same effort (and my lighter cyclocross is a bit faster still with 28 tires also, shifts faster, 9 speed vs 7 speed for my tourer, so closer ratios etc)

no matter what wheels and tires, max tire pressures really really makes a difference, especially with weight. Keep an eye on em!!

hope you take into consideration nixxing the backback. Is your bike able to take a front rack? Anyway, some more food for thought.

all the best