“Having the Constitution only formally owned by nine of the 13 colonies would not have been a ringing endorsement of the Union. Sure, it would have been legally bound by all states, but it would be less than optimal for a long term future.”
No. Not all states would have been legally bound; only the states ratifying would have been bound by the Constitution.
The problem has been identified: you have never read the Constitution or else you do not remember.
Still, your excursion does distract from the fact you are not prepared to confront the ugly truth that northern states easily incorporated slavery into the United States Constitution because it was in their economic and political best self interest.
And you obviously didn’t realize a “Perpetual Union” meant anything in Law.