Refute a first cause which most people would call G-d AKA Deism

[Esquilax]

5. This thing that must be the cause of the universe existence which is not composed of matter or spacetime is commonly called G-d.

You've skipped a few steps here, like showing us why the cause of the universe is a being, let alone something conscious, let alone one that created the universe through a willful act. Perhaps it was simply a random, natural thing that happened in a pre- Big Bang universe; no matter how rare such an occurrence could be, it would only need to happen once. Perhaps the universe is metastable and has always existed in one form or another. Perhaps multiverse theory is true and we spawned off of a parallel reality.

If you cannot refute Deism then I don't see why one should call themselves an Atheist.

I call myself an atheist because I don't know what reality's first cause is, nor if it even has one, yet I recognize that I'm not justified in leaping to any conclusions about it before I do know. Yes, the deist god is a tempting thing, in an unfalsifiable kind of way, but it's not nearly as natural a conclusion as you're making it out to be; you can't prove it, nor can you disprove the alternatives I've presented above.

That's because neither of us has sufficient information. What's so wrong about just admitting that?