I’m agnostic on whether or not Paul thought Adam was a “historical” person or whether he was using him as an established literary figure familiar to his audience and thought of him as symbolic. Regardless, I think Paul’s focus in Romans is on identity; Jewish identity (identity in Abraham), human identity (identity in Adam), and identity in Christ, and he plays with these ideas of identity and community membership throughout the letter.
So, I think the main point in bringing up Adam is to make the contrast between our identity “in Adam” (and the things we all share as a member of fallen humanity) and our new identity “in Christ,” I don’t think his intent was to explain the mechanics of some kind of literal sin transmission and removal process through biological inheritance.
Paul repeatedly talks about Gentile believers as being grafted into Abraham and heirs to the identity that comes from membership in the community of God’s people. The focus isn’t literal biological heritage back to Abraham, but spiritual heritage. Similarly with Christ, we aren’t biologically related and that has nothing to do with our redemption from our sinful human nature, it’s an identity that is taken up spiritually. Inheritance, adoption, “sonship,” and new birth are used metaphorically and symbolically throughout the Gospels and epistles. So I don’t think it is essential to the theology of the sinfulness of humanity that Adam be a historical person that we are biologically related to. I think the main thing is the spiritual status before God that identifying with Adam entails. And the Bible says we are all have that Adam/human identity, and we all sin and need to find our new identity in Christ to be reconciled to God.