A convention to propose amendments is not an open ended constitutional convention.
A fascinating topic. I recall a rather detailed thread in which Publius interviewed the late great Congressman Billybob (John Armour) on the topic. Pinging the Man...
Want to bet? Once you call a convention to propose amendments, it as open ended as the delegates want it to be. And you can bet that the first order of business will be the same as last time, i.e. making all proceedings behind closed doors.