Additionally, the idea that the serpent in the garden of eden is the devil is post-biblical.

First of all, the old testament doesn't really present the devil as a character, at all. Satan is a Hebrew word meaning "accuser", not a name of a specific entity.

For example, in the book of Job, a bunch of angels come before God, and "hasatan/the accuser" is among them; it seems more like it's a job title of one of the angels than as being the Christian idea of the devil.

Second of all, the new testament doesn't unambiguously call the serpent in the garden of eden the devil, literally anywhere.

Christians will often point to Revelation 20:2

He seized the dragon, that ancient serpent, who is the devil, or Satan, and bound him for a thousand years.

Although this is probably a reference to Leviathan instead, who is called both a dragon and a serpent elsewhere, and features in canaanite creation myths that are referenced elsewhere (e.g. Isaiah 27 closely mirrors some Ugaritic tablets we've found, just replacing Baal defeating Leviathan with God defeating Leviathan)

They'll also cite some verses calling the devil the father of lies, although this is fairly ambiguous. Particularly since the serpent doesn't actually say anything untrue in Genesis - it's at best a lie of omission, and saying it's a lie of omission presupposes that the serpent knew how God would punish everyone involved.