In reference perhaps especially to Gen 1:6, do any other ancient cosmogonies (creation myths) sufficiently resemble that of Genesis 1 as to be a plausible inspiration of it?

Or might they have a common source, anyway? The short answer is that other contemporaneous ancient cosmogonies—Egyptian, Sumerian, and Assyrian—all have enough and striking elements in common with the Genesis account that it is unlikely that they are utterly independent and unrelated. Just for example, one pagan god is said to plan and then speak certain things into existence. In another, the “Enuma Elish” myth of the ancient Babylonians, following a long and complicated back-story, it is a created god, Marduk, who becomes king of the universe and leader of the divine assembly, by defeating the goddess of salt water, Tiamat, which he divides (as some would say God does in 1:6). And the Egyptian god Atun is said to be the original and supreme god, and to have gotten the creation started. But he is by no means the only god, and there are many very important differences. Probably, there was one original tradition, which the Hebrew account gets most correct, being most sensible and being connected with a tradition deeply imbued with other qualities that compel belief.