You are driving at this idea that "echad" is different from the English number "one".
Not at all. In English the word "one" can be an adjective or a pronoun.
In Deuteronomy 6:4 "one" is an adjective modifying eternal being, but it does not specify the number of eternal beings nor does it specify which eternal being. In other words it does not support the premise there is only one eternal being. There is a word that means only one, but that is not the word used in Deuteronomy 6:4.
The New Testament is a commentary on the Hebrew Bible explaining things not obvious in the Hebrew Bible, for example the number of eternal beings in the days of Moses.
The NT makes clear that the Most High is the God of Israel's fathers, but it was not the Most High who brought Jacob's people out of Egypt.
The exodus of the people was commanded by the Most High but executed by Christ.
Christ is now the King of Israel on behalf of the Most High and is the person referred to in Deuteronomy 6:4.