Annexe has had a stab at it previously on the old forums in 2021, pre-Vecna but clearly well into the plot line so far:
But honestly, chasing a firm answer to this one won't result in anything that matches release order or level order. It involves time - it doesn't
have to make sense for either of those scenarios.
I blame chronoscope, frankly. You mess with time at your peril, that whole escapade at L5 probably cracked a hole right out of Eberron's reality, allowing Lolth to reach across the planes, amplyfing the effects of the Black Abbot's death, bringing the codex within Arretrakos' reach and so on, kicking off the whole shebang.
The driving forces of the story (i.e. the villains) experience that story in a completely different order to how your character does unless you wait to cap and do it all then, but I warn you even if you get all the answers to every question it's probably still a bit ouroboros because when the codex is shattered things echo
backwards in time and cause investigations and such to happen.
You'll need a flow chart to map it.
I think you are "meant" to experience it from the outside - i.e. when a given bit of the story intersects with your character's current point on their linear time line.
So if left to your own devices, it happens to
you in level order. But it
happens in an order probably somewhere close to the one Vryxnr posted. It
ends with Vecna - but the ripples of both that and whatever the true 'instigating event' that caused the codex to shatter in the first place go both forward and back through time
and across infinite plans. This is why remnants and champions still exist for everyone too, and in fact at this point always have. I mean, that and the fact the devs aren't going to rip that whole system out just cos a plot line concluded.
It's not clear (I don't think) in the plot yet as to
what caused the pages to fragment into remnants in the first place, which might tell us what plot line led to that for a true 'beginning'.
As I say, when in doubt, blame Chronoscope.
In terms of playing it 'as intended' you probably can't cos I very much doubt each step on the journey (including dialogue and notes etc) was written with the full story line in place. The 'intended' way to play it is to do it entirely out of sequence and piece it together because in linear terms Eberron experienced it out of sequence, I think.
What
you want (what I want too, frankly) is to experience it in "protagonist order". Instigating incident through to ending - whereas experiencing it how you're 'meant' to (i.e. as the codex storyline intersects in your character's linear timeline, out of "order") , you just sort of feel like your role is as an observer of grander schemes.
