Water is not light.
yes, but the key bit in there is 'when all other capacity has been maxed out'
Fork cages are normally used for bulky and light, but some people have been known to use them for a couple of additional bottles when on a long trip, and the frame bag is full, all other cages used, especially since sleeping bags and tents can be too big to go on forks and still leave clearance in singletrack, but a bottle tucked behind the leg is fine, it's a compromise and they really are a last resort!
Unfortunately bikepacking does not work for me ...
An 8 day trip ... means 10 days of food at 1kg per day + 3.8 litres of water (I expect to get water during the day... but no food).
+ camping gear, cooking gear, + a little clothing, camera, PLB, GPS, maps, towel, soap, first aid kit ...
there is just too much of it.
That's another example of starting to confuse bikepacking with a normal tour. If you're away for 8 days, in a place where you cannot acquire food along the way so have to carry it all from the beginning, then you're already into the grey area of it not being a normal mountain bike ride with a few sleeps, but being into the realms of a small tour.
Also, a lot of it comes down to your own personal level of 'comfort' in terms of how minimal you go, for example you say 'cooking gear' now I don;t know what you take, but for a committed bikepacker cooking gear might consists of:
- a 750ml mug
- a micro stove (or homemade one from a drinks can)
- a spork
both of the second items fit in the first so the space and weight requirements are tiny, again, I don;t know about your 'camping gear' but a committed bikepacker going out in spring/summer might take:
- a lightweight (600g) sleeping bag
- a bivi bag (350g)
- a small mat (400g)
- a tarp (300g) (no poles, you use the bike for that)
That's your entire shelter AND sleeping system in roughly the same weight as a one man tent alone, all of which can pack into a single seatpost mounted pack and still leave room to spare for some extra night time layers or some food. Do you really used 3.8l of water per day? I normally work with 2x750ml bottles, if you can refill along the way you only need enough to get between refills (+ a reserve) so 5 bottles seems a bit overkill, but it depends where you are, civilisation or wilderness, if you're bikepacking then you're probably expecting to pass through a town or village at least once or twice a day, if you're in a place when you won't then your requirements will be very different and I don;t think anyone would consider ultra light bike packing for that kind of trip.
Sorry if I'm saying stuff you already know, but I'm just trying to highlight that it's often not that 'bikepacking doesn't work for you', but that you're not 'doing bikepacking'
When I'm doing a different kind of ride I take a different approach, panniers, tent, bigger stove, a couple of pots, more clothes etc, and use different luggage accordingly, it really is about horses for courses, a single or couple of nights away on the moors is a different kettle of fish to 8 days away from civilisation.