One thing I think I've misunderstood about "Getting Things Done" is the expectation that it asks for uni-stream task management. That may be something that the official materials demand, and that may be accurate to the system as it is practiced, but I don't really believe it's necessary to the essentials of GTD.
GTD must have reference to a "uni-stream" in relation to time and that's basically it. Everything must all come out of the same extension of lifetime that's available to you.
As far as record-keeping, I think it's been overstated how much GTD relies on a single stream of evidence and documentation. I believe that if you're a functional adult, you can probably read and write into two or three different "area" notebooks and recall from there effectively.
I think that the voices that will prioritize the uni-stream are most committed to having automated recall, like automatic reminders, and who are most skeptical about human memory.
You know the old line from Emerson? "Every observation about the world is a confession of one's character."
As for myself, I hate being minded by digital systems and I'm doing pretty great with my meat memory. I'm not an execrable sinner held over the flames of ADHD by the acausal robot god-calender.
I think the version of GTD that I'd really love would be Tao instead of Zen.
I'm not a samurai with an elegant and seamless integration of purpose and energy.
I'm a mossy rock disintegrating under a waterfall and my spontaneous inconsistency is the authentic source of my mossy genius.
To my undrstanding "Getting Things Done" transforms problems into a request for one of these 3 inner resources:
1. Choice - which is no more than working your way through the decision tree
2. Courage - which is no more or less than engaging at any moment with the present task
3. Memory - which is no more than writing things down and reading them later