What’s the point of a third vote to simply reaffirm what’s already implicit through both the first two votes? That vote is what it creates this never-ending loop where if voters refuse the constitution the person they picked chose, at least one referendum decision will eventually have to be disrespected.
I suppose the excess of democracy is to make sure whatever is written is the most consensual as possible? In that case just come up with a bland generic small text that all sides can be ok with and get this over with.
Surely the definitive lesson from your own Centrão is how little you can trust an elected legislature to do what they promised for the voters. You don't even need to imagine a Centrão dominated constitutional assembly, you've already had one in '88. An excess of democracy is to be expected given past experiences of the opposite.