Experience can't be all though, or can it?  Something about that seems not right to me and yet, I have no idea what...

The major problem with Descartes, which you're pointing if i understand you well enough, is that he ends his hyperbolic doubt with doubt itself. But doubt is still a concept, it is still something that refers to something else (the language you use to express it, you doubt something...) therefore you can never have pure thoughts. Experience is experience of something which you can't get rid of.
As for the BBT, i am currently reading his paper (https://rsbakker.wordpress.com/essay-archive/the-last-magic-show-a-blind-brain-theory-of-the-appearance-of-consciousness/), which is quite "velu" as we would say here. Thx for bringing this

Here's a theory (didn't see it mentioned so far, maybe already discussed ?):
TLDR : Kehllus is to become the No-God.

Why : Sorry but a bit of Mind-Body problem explanation is needed here. What i understood so far of the book's world is that it basically works as a Spinoza viewed ours. Id est free will is a lie (we only think ourselves free because we don't see what moves us), everything is a chain of causes and effects, AND there is actual difference between the physical world (PW) and the let's call it "soul world" (SW) where minds/thoughts/souls (whichever you prefer) live. How come we see connection between what we think in our soul (or mind) and the physical world ? Because God made the world so that the sequences in our soul coincide with those in the physical world, there is kind of the same logic in both and understanding one is understanding the other and both are determined on parallel paths.
Thus there is 2 major sins you can do (impossible for Spinoza but Bakker can do what he wants ), which both break the link PW/SW :
- you could apply on the PW the meanings born from your soul (the SW), therefore making something happen in the material world that wasn't cause nor can be explained by something in it. It's called magic, wizardry.
- or you could make something happen from your understanding of the PW (that is science) which has no SW equivalent. Let's dwell on this one : doing so is doing something that has literally no meaning, no soul. The first exemple of this is the Tekne, the second is the Dunyain program.
What the Survivor understood is the deep difference between knowing "how" and knowing "why", and that the Dunyain way is a dead-end in this regard (in France we say : science is knowing that tomatoes are fruits, wisdom is knowing  not to put them in a fruit-salad).
Have the Inchoroi created the Dunyain sect or not, they both lead to the same thing (the dunyains being far more efficient) : the creation of something with the powers of a god but without any SW equivalent, therefore that cannot be seen from the SW and the gods.

Sorry for the english, not mother tongue