To which we could add that God's essential qualities ─ omnipotence, omniscience, as you mention, omnipresence and perfection often included ─ are, as far as our knowledge of reality is concerned, all imaginary, all lacking even a hypothetical basis by which they could be elements of reality.The problem with defining atheism succinctly is that there are many kinds of theism of various amounts of definitional rigor.
For instance if someone believes in a god that specifically has the property "flooded the world mere thousands of years ago," this is a concept of a god that can easily be refuted with empirical evidence. It's easy to say that this god does not exist.
However if someone defines their god much more nebulously, such as if they only say "god is an omnipotent, omniscient being that created the universe," this is much more difficult to outright refute. So a reasonable person in this instance doesn't say "this thing does not exist," they may merely sit on the position that they're not convinced it does exist until more evidence is forthcoming.
For these reasons, the most widely encompassing definition of atheism would simply be to at least abstain from affirming any theistic propositions as true.
But all consistent with the observation that the only way supernatural beings and magic are known to exist is as concepts and things imagined in individual brains.