I agree with you that the preamble is the purpose statement of the US Constitution. Legal gurus from the establishment will say we’re all wet, that it’s only a flowery opening. In my view, no interpretation can violate the purpose statement.
Some time ago, my brilliant wife noted that in a very real sense, the preamble, or statement of purpose, IS the Constitution. Those who want to pretend it doesn’t matter are like somebody arguing that the foundation is not part of a building.