The text keeps repeating “and he died,” and this formula is not employed in later genealogies. Why? Surely simply saying that Adam lived 930 years (Gen 5:5) sufficed to imply that he died?

First of all, there was one patriarch who is notable in that the text does not say he died: Enoch. Instead, because he walked with God, he was “translated” or removed to Heaven. The others, however, went down to the grave. This is important because, as obvious as this would be without being told, the author is underscoring, with a stark and grim poetic beauty, the truth and faithfulness of God’s promise that the wages of Adam’s sin would indeed be death.