Or is it as Chief Justice Marshall pointed out, that the authors of the Constitution gave the broad outlines and meant for much of the meaning to be interpreted based on those outlines. So if the Constitution gives Congress the power to admit states in the first place and to approve any change in their status or border once they're in then I don't think it's any real stretch to conclude that Congressional approval is needed for them to leave. If states cannot take steps which impact the interests and well-being of other states without consent of Congress while they are a part of the U.S. then why should we assume they can take those steps while leaving? To my way of thinking assuming that the founders meant for the Constitution to be used as a club to beat up remaining states if a particular state wanted to leave is too far fetched to believe.
But I note that you don't address why the Framers mysteriously chose to remain silent on this issue when they had spoken loud and clear in the Articles. Assuming, arguendo, that "it's not a stretch to conclude" that Congressional approval is needed for secession, what possible reason would the Framers have for not explicitly stating that states did not have the power to secede? After all, as you stated, the Framers made a laundry list of powers vis-a-vis state admission, division, and borders. Why would they go silent on secession?