But the origin of seraphim, and thus Satan taking the form of the snake in Gen 3:1, might have been with some pagan flying snakes, no?

Serpents, indeed even flying serpents, were a feature of ancient pagan cults, but the mere fact that the word seraphim was used hardly means the angelic beings were took serpentine form. Indeed, as some like to point out, there are other uses of “flying fiery serpent” (שָׂרָ֥ף מְעוֹפֵֽף׃ or saraph me’owpeph; Isa. 14:29). But the origin of Isaiah 6’s concept of a seraph, if you actually believe the Bible, is with the creatures called the seraphim themselves; you look for another origin only if you believe that explanation lacks credibility. Never anywhere in the text of the Bible is there the slightest indication that any of the inhabitants of heaven take a serpentine form, apart from the mere name seraphim; and again, in the one place where those beings are described, they are not described as snaky at all. The point is that the Hebrews who wrote and read the Bible clearly did not conceive of seraphim as snakes, whatever role snakes might have played in their notions of other-worldly realms. Therefore, the correct explanation of the fact that Satan took the form of a snake is probably not that he was previous a (snake-like) seraph.