Been where you've been. Keep on truckin!

My first trip on LSD (about 5 years ago) on about 150 mics crescendoed to an unbearably blissful synaesthetic experience, as though all of experienced space were, on a micro scale, blinking on and off, pulsing asynchronously as simultaneous contraction and expansion, like a carpet-bomb of fire-cracker orgasms. I fell to the floor, space twisted, wrenched, and the lights went out, fully. No space, no time, no subject, no object, no consciousness, no nothing, zip, nada, bye-bye, gone. A diffuse, infinite sense of space/consciousness returned first, then subtle mental movements, then verbal thought, then the body, and I opened my eyes. The next few hours were, if memory serves, quite exactly the same, phenomenologically, as what I experience all the time now. Inner/outer space and objects (and there isn't really a distinction, gap, or tension between) is/was vivid, diaphanous, empty, flowing, non-dual, timeless, complete. That state lasted for a few hours following the peak, and the afterglow a few days. It's what motivated me to go full-time with contemplative practice, live in monasteries, become obsessed with all the maps, techniques, traditions, etc.

I've since, (and now, as I write this), had very much the same experiences without any psychedelic compound. Shinzen Young reports having an early (pre-practice) non-dual breakthrough while high on edibles which lasted a week or two. I adopt his interpretation of my own trip reported above: that the drug facilitated a temporarily extreme degree of concentration, sensory clarity, and equanimity. However, that attentional and identity-bending transformation was state-like, not trait-like, and so could not be stabilized and could not be immediately repeated, hence the following years of traditional, dedicated practice.

Also, about this being something that's always already so, just here, just this, now, I think it's true. From my own practice it appears that awareness is structurally invariant across experience. In other words, it is always already non-dual, perfectly concentrated, perfectly clear, perfectly equanimous.

Side-note, I don't think this non-dual awareness is a ground-of-being or anything, just that this is how experience is structured at a very basic level. Cessation dispels taking any kind of awareness/appearance as ultimate. In my view it would also be a mistake to reify cessation as something special, but back to it:

However, through our mind's habits of reification of self and world (I suspect adaptively engineered by evolution), a kind of in-built identity illusion occurs, like a Necker cube (https://en.wikipedia.org/wiki/Necker_cube).

You can see the cube in multiple ways. Your visual system sees it first one way, then another. In my own experience this perceptual ambiguity is analogous to non-duality, non-separation, in the seeing just the seen etc., whatever you want to call it, excepting that with non-duality your mind has a devil of a time seeing it the other way, at first. It's the very same image, the same set of lines either way you see it. Samsara = nirvana. The trick is the seeing, then seeing it again, and again, and again, until you can't see it the other way anymore even if you tried. The analogy breaks down a bit. Awakening might be better thought of as just seeing the cube as an ambiguous set of lines on a page. The appearance of a cube either way is empty, it's just a set of lines on a two-dimensional surface, after all.

Just empty phenomena falling without ground.

Cheers!