Q: Was the Big Bang really a blast of concentrated self-belief as the first ever sentient entity, a tulpa or “thoughtform”, willed itself into being?
A: Yes, but not the BigBang, it was later—when Eukaryotic life evolved it gained the ability to access quantum computation (Penrose / Hameroff, microtubules, Orch Or etc) and for the first observer the universe worked as described in ‘The Secret’, i.e. anything it willed, it got… but as the population grew the evolution of the universe became a best-fit confabulation of all will. As it still is.
As for the Big Bang: far as I understand, when the ‘inflation’ period of superluminal expansion ended and conventional expansion began, the universe was about a meter across. Annnddd…. one might hazard that ‘Inflation’ is just a mathematical hand-wavy kludge anyway. So whatever started expanding at the beginning of creation, something that curled up would fit in a ball a meter across, was obviously a person warping through hyperspace into a virgin region between universes and that kicked everything off.
(Sci-fi story stuff, don’t take seriously… but you never know, he was probably called Adam or something.)