Have you considered something like this:

https://ultimatedirection.com/pb-adventure-vest-3-0/

Water bottle pocket is on the front strap, so no contortions to get it. It's only 16L, but I'd think that's enough for a day pack. This is a runner's pack, developed (I believe) by Peter Bakwin. Weighs less than 14 oz. If interested, there's someone (not me) selling one for $75.