I see where you're going with this and I understand your concept, but you're incorrect on the topic. The Nephilum are the children of fallen angels and human females, this is per Genesis 6. There are at least 2 or three additional times in the OT where beings with 6 fingers and 6 toes are mentioned in scripture. Goliath was a descendant of these, along with his brothers. If you read scripture carefully you'll notice other times when the Israelites came across giant fruits along with giant people.
There are also published works that speak of the Native Americans talking about giants with 6 fingers and six toes on each hand and foot. They have this recorded and dated at the same time the OT was written. So two different peoples writing about the same thing on opposite sides of the earth? No such thing as coincidence.
Don't dig in too deeply here, because you're doing it based on tissue paper.
First, the "Native Americans", the Indians, had no written language. There is nothing whatever written and dated from OT times among the American Indians, because the American Indians had no written language until the arrival of the Europeans. There are no written texts of the American Indians before that, because the Indians never developed an alphabet or a hieroglyphic system. There are a few cave paintings, and some notch stuff on wood, but nothing more.
So no, there is no "recorded and dated at the same time the OT was written" written texts by American Indians. They do not exist. The Indians had no writing system until the 1500s. If someone is telling you otherwise, let him publish the actual PICTURES of these texts, lots and lots of them.
Anybody can write about anything. Except the American Indians, they could not write anything until after 1500, because they never developed a written language.
Genesis 6 does indeed say that the NPLYM ("NePiLYM") are the offspring of "sons of God" - angels - (it never says "fallen") and human females. And it says they (or at least some of them) were the "great men of old", in other words heroes.
The concept that these angels were FALLEN because of these matings is not in Genesis. It is not in the Hebrew Old Testament. It is in the Greek Old Testament, in the Book of Enoch, which is part of the Ethiopian Orthodox Canon, but is not part of the Catholic, Eastern Orthodox, Protestant or Jewish canons.
Jude references Enoch, which would indicate that the FALLEN nature of these angels was indicated by Jude.
But if you stick within the Protestant/Catholic/Eastern Orthodox Canon, you don't have Enoch, and if you stick to the Old Testament, you cannot assert that the angels who mated with humans before and after the Flood were "fallen", or "wicked". Because the text never says it.
Nor does the text ever say that Goliath was a descendant of angels, or a nephil. It merely states that he was tall. People have interpreted that to mean he was a nephil, but that is pure speculation unsupported by anything in the text other than reference to his height. Kobe Bryant is also tall, and is a prominent "hero" of our day to many. Is he a nephil? It seems unlikely.
The Rephaim were described as very tall. King Og of Bashan with his 10 foot (or so) bed was one of them. But the Bible never actually uses the word "Nephilim" in conjunction with the word "Rephaim". It's a leap of logic readers present and past have taken, but it's not actually in the text.
In the Torah, as the Israelites approach Canaan, spies are sent out. They return with the message that the land is unconquerable, and say they met the Rephaim there, beside whom they were like grasshoppers. But Joshua and Caleb don't agree that the land is unconquerable. The context of this information is important. The spies who report the huge stature of these Canaanites may well be exaggerating, because they are trying to discourage the Israelites from attacking Canaan. The Rephaim may well have been very large of stature (and of course they could even have been giants, as reported, and even outright Nephilim), but to take at full face value as true the reports of men who are trying to actively discourage the Israelites from doing what they were supposed to do is not required. In fact, because the Israelites listened to those - probably much exaggerated - reports and decided not to enter Canaan on account of them, and God punished them for that by making them spend another 40 years.
Indeed, when the Israelites actually DID invade Canaan and Bashan, a few years later, they found large men (Og, and later still, Goliath), but they never encountered any men who made the Israelites look like grasshoppers alongside of them.
So, faced with these passages about the giant Rephaim, we cannot simply take them at face value for at least two reasons:
(1) they were being given by FALSE spies who exaggerated in order to discourage the Israelites from correct action, and
(2) When the Israelites did the right thing and invaded, they found big men, but they didn't find any towering Titans beside whom they were "as grasshoppers".
We can choose to take them at face value, if we want to, but even if we do, the word "Nephilim" does not appear in the accounts of the Rephaim, or of Goliath.
Moving on, when you mention that there are two or three ADDITIONAL times in which six-fingered men are discussed, that may be so, but the word "additional" is misplaced: there's nothing in the account of the Nephilim in Genesis that gives any suggestion that they had six fingers or six toes. The references to the six-fingered men is to a specific town, the Philistine town of Gath, whence Goliath hailed. That would indicate that there was an hereditary six-fingered trait there, but it doesn't tell us anything about the Nephilim as such, because the Nephilim are never mentioned at Gath.
So, to not put too fine a point on it, the "careful reading" of Scripture you have recommended - if you DON'T include Enoch as Scripture - does not lead where you have suggested. Nephilim does not mean giant. The Rephaim and some Gathites were big men, but the text does not call them Nephilim. Two different things are described as the English word "giant", but that's not actually SCRIPTURE, it's a mistranslation of two obscure Hebrew words, combining two thoughts - into one, and ascribing something to that one word that the Hebrew words don't contain.
There are some really big men in the Bible. And there are offspring of human women and heavenly beings in the Bible, Jesus being the most prominent, and the Nephilim being the others. They are not described as BIG, they're described as GREAT, as heroes, men of renown. Hercules wasn't 30 feet tall. Neither was Achilles. Demi-gods COULD be huge (Polyphemus the cyclops, for example), but their name "demi-god" doesn't mean "giant", and neither does the word "Nephil".
Equating the Rephaim with the Nephilim is simply an assertion based on an inference, not Scripture.
Unless Enoch is Scripture.
If Enoch is Scripture, neither Og nor Goliath were Nephilim, for Enoch tells us that the Nephilim were 300 cubits, the same length as the ark. That's about 600 feet tall. It's more complicated than that, for the Amharic version of Enoch tells us this, but the Greek version gives different races brought forth by the union of women and angels, of which the Nephalim were one.
If one adds in the Books of Jubilees and of Jasher, both of which are in the Ethiopian canon, a larger picture of the Nephilim emerges. And frankly, if one does that, one can begin to come to the conclusion that perhaps the great therapod DINOSAURS were Nephilim, and the Flood destroyed THEM, and was INTENDED to.
There's plenty of Scripture that will give you all sorts of details about the Nephilim and the Flood and the antediluvian world, but you'll have to go to the Ethiopian Orthodox to have them in your canon.
If you exclude those books and everything contained in them from consideration, from just the Bible itself you can't get all that far. And, thing is, even if you INCLUDE Enoch, Jasher and Jubilees, there is still the problem with the mistranslation of the Hebrew word Nephilim into Greek as "giant", because the word simply does not MEAN "giant". Nor does the word "Rephaim".
NePLYM is - the seed or generation of the mouth that leads to chaos.
RePYM is - the head (or summit) of the mouth that leads to chaos.
One speaks of a generation, a bloodline. The other speaks of the leaders, the heads. Same thing - but physical stature is not the key to the word, unless one views "summit" or "head" as meaning "promontory".
As I say, one can INFER things about the Repym/Neplym from the Catholic/E.O./Protestant and Jewish canon. And one can find some confirmation of that in the books mentioned in their canons but actually contained in the Ethiopic canon.
If you want your giants at their full stature, go with the Ethiopians.