I started this project in august (pattern and material list) / september (actual sewing) and finally, this evening, it is usable!
It wasn't the only project running (not even the only *sewing* project), but still it took enough time that I'm glad I can finally use it, even if right now it doesn't really have any real advantage over what I could have bought from any store for less money than I spent on materials (but I have many leftovers — and plans to use at least part of them)
It also was pretty challenging, both for my skills and for my home sewing machine, and there are a few things that could have been done better (and maybe they will, in the next backpack I'll make?)
But is it really done? No, that webbing in the front (and sides) is there to attach extensions, and I have at least a couple planned (one easy, that I will do soon, and another complex one that will wait until I've recovered from the project fatigue with something completely different).
I'm also still waiting for the buckles that will close the compression straps in the front (not that they are really needed now that there are no extensions to keep compressed), but they are details I can take care of later.
P.S. I took pictures and notes, and the pattern is already on git, but a full tutorial will have to wait, probably even months, as now I have a bit of sewing backlog.