I think that the Constitution provides a guideline for that. The most logical method for states leaving the Union would be the same manner in which states joined the Union, through a majority vote in both houses of Congress.
Some States were recognized and joined the Union under Article IV, as you say, through admission by Congress, but the original 13 are, it seems to me, the acid test: they came in under Article VII, by conventions of the People assembled.
We obviously are NOT going to agree about the nature of the People. You, fully realizing what is at stake polemically, will never agree that the People are the States are the Sovereigns. This is dishonest, and IMHO, a loser's scheming. You want the Government to be sovereign "for" you......which works as long as the side you favor has working majorities in the Congress, and as long as the Executive and the Judiciary are still listening to the Congress, which is nowadays a debatable proposition.
You've decided to stand with Lincoln on his fictions, and take your chances that his successors will be nice to you. Good luck -- but meanwhile, you are threatening by your supinity before Hamiltonian duckspeakers to ruin all our freedoms. So I hope you'll understand why I'll be staying mad at you. Unions can't maintain a living wage if members break ranks and knuckle under, and dealing with tyrants is just like dealing with gold-plated SOBs -- and for exactly the same reasons. The servility of a few condemns all the rest. Hope you're happy -- not.