God could very well have made the whole population of Homo sapiens (several millions by the end of the Neolithic) in the image of God at once, ordering each of them to love God and the others, and making each human being accountable for the life of another human being.
However, God in his omniscience, took account of the possibility that humans sin and the sinners disregard that God let them on earth to give them opportunity to repent and reach salvation, to the extreme of living as if God were obliged to accept their sinful ways. God foresaw that humankind could become corrupt and full of violence, and a big remedial would be needed to correct this wicked mentality and avoid “hell on earth”. The flood stands for this remedial. Although sin is a liability, the focus of God’s attention and divine plan is not sin but love: God wants to preserve that people can be redeemed.
This is the message Jesus Christ and the Apostle Peter convey in the context of their teaching about the End Times, and this also what you suggest about the meaning of the flood:
Nonetheless, to avoid killing millions of people, God preferred to make a “first-version of accountable humans” reduced to a little population (i.e.: He transformed only a segment of Homo sapiens into “accountable image bearers”). This primeval population increased in number, possibly to several hundreds of thousands, who chose in fact the path of corruption and violence (Genesis 6: 5-12), and was deleted by the flood (Noah and his family excepted).
At the end of the flood God definitely re-create the humankind by making all Homo sapiens (up to possibly 14 millions) into a population of human beings accountable to God: “From each human being I [God] will demand an accounting for the life of another human being […] for in the image of God has God made mankind.” (Genesis 9:5-6).
By proclaiming this universal prohibition of homicide in Genesis 9:5-6 God highlights again that “God loves humans” and wants that humans love and respect each other. From this moment till today ALL peoples on earth share in the dignity of being in the image of God.
I hope the preceding clarifications show that, like you, I think that “the point of the narrative is to teach theology”.
Nonetheless I also think that the narrative of the flood is historical and describes a global catastrophe that destroyed all human beings living at this moment (Noah and his family excepted), where by human beings I mean humans who God had made in the image of God and morally accountable for their deeds.
Notice that catastrophic floods capable of killing hundreds of thousands are common, as @mitchellmckain has fittingly stated in a previous post:
In summary:
I respect your belief that the flood narrative “does not teach history” and “does not describe a literal global catastrophe”, but I dare also to ask in this respect:
Do you agree that my explanation above fits well with revelation and is not discarded by science?