There is no official LDS doctrine on what earth God lived on. The LDS believe in continuing revelation so this may be revealed at a later time.
There are plenty of questions that we currently don't have the answer to, but IMO those questions aren't pertinent to one's salvation and so don't matter currently.
17 And I said unto him: I know that he loveth his children; nevertheless, I do not know the meaning of all things.
See also You Know Enough, Elder Neil L Anderson
related SE post on exaltation please read this post to gain better context of below
Opinion:
From OP post
...he was once a man like us...
God has a god of his own, just like we have a God who created worlds, and we can also become gods(but do not replace God). God's god is NOT worshipped by mankind. His worlds are his own/separate.
Think of it as a family. You can become a father(god), you have a father(God), and your father has a father(his god). Your father's father is not your father, just as you becoming a father(exaltation) does not replace/reduce your father.