The existence of Satan doesn’t undermine monotheism any more than the existence of angels does. Satan is a created being that was once an angel, but fell at some point in the distant past.
The problem within your argument is that in Job chapter 1 and 2 is Satan appears before God in the presence of the sons of God. It says in those chapters that there was a particular time long ago when the sons of God appeared before the Lord (God) in heaven and Satan came also WITH THEM (the sons of God).
Thus, three distinct sets. 1.) God, whom the 2.) sons of God (angels) appeared before, with 3.) Satan in their (the sons of God) midst.
Thus, Satan is a separate, created being, one apart from God.
Job is usually regarded as fictional by most rabbinical authorities, and I suspect, a lot of Christian pastors.