These are your two strangest proof points.
When you write this,
 The parallelism between day 2 and day 5, this works best if birds fly ‘in’ the created space, not below it.
You imply that without a celestial ocean, day 2 and day 5 become incredibly compelling. Hardly. What even tempts you to think this?
Setting aside the idea that we can dismiss all sorts of references to a different kind of firmament, based on the idea that the aesthetic appreciation of poetic symmetry must somehow be determinative in the proper unveiling of the Bible’s unveiling of the Earth’s physical nature …
where exactly is all this symmetry?
In Day 2, we read about the waters. And we read about the Waters that are divided. There is nothing at all about the land, or the Earth. It is all about the Sky and the Waters. Ok. Good so far.
Here are the living things of Day 5, according to the Revised Standard Version!:
And God said,
(a1) "Let the waters bring forth swarms of living creatures,
(a2) let birds fly above the earth across the firmament of the heavens." (Reggie says “in the firmament”)
[Waters mentioned, and the sky mentioned.]
So God created
(b1) the great sea monsters and every living creature that moves - - with[in] which the waters swarm
. . . and
(b2) every winged bird according to its kind. And God saw that it was good.
[Waters mentioned, but no mention of the sky]
And God blessed them, saying,
(c1) "Be fruitful and multiply and fill the waters in the seas,
(c2) let birds multiply on the earth."
[Waters mentioned, and the birds are mentioned … on the Earth?]
And there was evening and there was morning, a fifth day.
The first thing we notice is that unless you are reading King James, the birds don’t fly in the firmament, they fly across the firmament. So your entire premise based on in becomes dubious from the get-go.
Secondly, Section A establishes that Birds appear out of nowhere ! Or you can conclude that Birds appear out of the Ocean. This is an immediate breakdown in the proposal that there is any symmetry at all.
For there to have been symmetry, there would have needed to be waters - - And Land - - in order to talk about the creatures of the Sea and the Creatures that Flew above their dwellings on land. Birds don’t have dwellings in the sea. So introducing them is, frankly, a bit of a bungle (in respect to section A).
Section B? Symmetry? Only strange or incomplete symmetry. We have sea creatures that swarm the water … and then we have birds. But it doesn’t say where the birds are. Are they flying? Or are they nesting? We have to assume the Birds are flying … in order to complete the symmetry.
How about Section C?
Well, we have the fruitful multiplication of the creatures filling the waters… and birds filling ?.. filling the ?
Nope… not filling the sky! The birds multiply on the Earth!
Day 2 says nothing about the Earth. Only about the Sea and the Sky.
Section A of Day 5 says birds fill the sky.
Section B of Day 5 says something about the waters… and finally,
Section C of Day 5 says waters and the Earth!
If there is no symmetry in Day 5, how can you use the even greater asymmetry between Day 2 and Day 5 as evidence of anything?
Your claim for symmetry is not only rejected as irrelevant, it is rejected for non-existence.
… without even mentioning that there is actually no Hebrew word for the word “in” that you keep injecting into the “the firmament”.
Thus I find it ironic that you then ask why there is no time given to discuss the atmosphere below the firmament! I would turn that question on its head: if the firmament is the atmosphere why doesn’t your so-called symmetry spend more time with it? Instead, the waters of the sea creatures are mentioned three times, but the sky for the birds is only mentioned once, which is exactly the same number of times Day 5 mentions the Earth of the Birds (section C)!
And yet day 2 mentions the waters and the sky, and the fish and the birds … so, symmetrically, and given “equal time for opposing states of matter”, day 5 should have mentioned the sky in section B (instead of silence) and should have mentioned the sky in section C (instead of mentioning the Earth, which isn’t mentioned in Day 2 at all).
Talking about the compelling nature of the Symmetry of day 2 and day 5 is like describing the compelling nature of Shakespeare’s sonnet on praising the beauty of a woman in comparison to the mixed blessings of a Summer’s Day - - which must suggest, then, that Shakespeare thought the woman was as beautiful as the Moon - - even though the moon is not mentioned once in that sonnet.
In the matter of Day 2 and Day 5 of Genesis, you cannot build symmetry on a single mention of the sky!
The symmetry is with the animals named… not in where they spend their time! The latter doesn’t even well accommodate such a thing, since unlike fish, birds must spend a great deal of time not in the Air. Whereas Fish spend 100% of their time in the waters.