I see the firmament as the separation between the physical and the spiritual realms - not a geometric boundary in space/time.
Additionally, because Genesis begins by saying He created heaven and earth (and because of Christian passages which you would not embrace) --- we ought to view Genesis 1 as speaking to the creation of all that there is and not just the physical realm.
As with the Temple and the Ark, things which happen in the physical realm are a model of the real thing which exists in eternity. I see the Garden of Eden in the same light the real Garden is paradise.
Therefore, Genesis 2 and 3 speak of events which are concurrently transpiring in eternity culminating with Adam and Eve being banished to mortality. That's when I see Adamic man entering the physical realm in the form of a human being. That constitutes the Fall, when death entered the physical realm, i.e. spiritual Adamic man must now die. What made the difference between Adamic man and all the other men who were on earth was the neshama the breath of God.
At that point the narrative of Genesis, the aspect changes to Adamic man and therefore, time passing becomes relative to our space/time coordinates. About 6000 years have transpired on Adams clock.
If we follow the archeological evidence mans desire to achieve immortality, the use of tools, personal adornment, community living, commerce, weapons, star gazing, household "gods", "super" men and the ilk all commence and flow along the Adamic man timeline and geography.
And at about 2350 b.c. (which matches approximately the time of Noah) - virtually every center of such civilization in the entire world was wiped out by a catastrophic event. Some attribute this to cosmic debris, some to earthquakes but it is characterized by massive flooding and destruction, even changing fertile ground to desert.
AG, have you read any of the gnostic gospels? There seems to be a strong similarity between what you wrote above and the beliefs of some of the gnostic sects, in particular the idea of the Fall as bondage to the physical realm. The gnostics were on the fringes of early Christianity; they had some influence on its development, while at the same time the more extreme gnostics were anathematized.