“1/3 of the states joined the Union during or after the Civil War, when it was pretty clear secession wouldn’t fly.”
Well, secession may not have been, or may not be successful, but the right to secede was not precluded.
A supposed constitutional right for a state to secede did not occur to people until the 1820s. It never even came up during the convention or ratification.
It is precluded by the nature of the term “constitution” which was the FOUNDATION of the Union and could only be changed by the amendment process.