Moses is the one who penned Genesis and was as well the one God elected to teach these unlearned newly free slaves out of Egypt. And IF Moses was going to be teaching them about how long Methuselah 'lived' and how that by Genesis 6 the Heavenly Father said I will shorten the time span of flesh living upon this earth. Then that would be by necessity mean the difference in what a day with the LORD means versus what a day means to flesh beings.
And that would also mean that Moses who penned Genesis 1:1 and 2 would have had to explain to these unlearned in the WORD that there had been two different floods for two totally different reasons.
The Bible does NOT say sin first took place with 'flesh' it say that 'the' Adam was the first flesh to sin.
We in these flesh bodies were NOT in flesh bodies when Satan rebelled. At Christ tells Nicodemus in John 3 that just to 'see' the kingdom of God one must be born from above. Christ did not say that all that are born in flesh would enter the kingdom of God but they would see it. As we are told in Revelation about those 7,000 that would be instantly killed at the return of Christ. Those are the ones who polluted the 'daughters' of 'the' Adam in Genesis 6 and those who the book of Jude among other places describe.
Those dinos left the evidence that this earth is millions upon millions of years old, else Noah would have been required to have built a bigger ship to house them when the polluted bloodline to Christ was removed from this earth.
Personally, though I tend to fall in with the YEC crowd, I believe there is room for debate on some of the points you are bringing up. The well-known 6,000 year chronology of the Bible only takes us back to Adam, the first man. Beyond that, we have a few paragraphs of Genesis, and some other assorted verses with which to try to patch together what occured before the era of man.
It seems obvious to me that the angels were created before man, yet I do not know exactly when this occurred. Yet, as Satan appears in the Garden, seemingly fulfilling the role of “the Accuser” in the earliest days of man, it seems sensible to believe his fall occured before the 6 days of creation, which I believe are literal days and not eras.
If there seems to be some history regarding God’s angelic creation that is not yet fully revealed to us, and if we can make a reasonable assumption from Scripture that some important events involved in that creation occured before the era of man, then I don’t see grounds to condemn anyone simply for speculating about the possibilities of previous creations or cataclysms.
However, I think, even if one theorizs some life may have existed before the Genesis creation event, that life must surely (excluding the angels) have been extinguished before the current creation. So, I believe those who attempt to use “gap theory” to give a more ancient lineage to man or beasts currently living, must be mistaken in that regard.