Could Texas leave through a constitutional amendment?

Yes, that path exists, and tracing it exposes how weak the "Texas can never leave" claim really is. The same Article V that lets the states rewrite the Constitution can be used to provide for a state's departure, and the logic of that power reaches further than most people realize.

Article V puts the states in charge of the Constitution itself

Article V gives the states the power to amend the Constitution, and there is almost no limit on what an amendment can do. Three-quarters of the states, thirty-eight of them, can change any provision they choose. They can add powers, strip powers, or rewrite the structure of the federal government from top to bottom. The Constitution has been amended twenty-seven times, including the Twenty-first Amendment, which repealed the Eighteenth and ended Prohibition outright. Amendments can undo amendments. The states hold that pen.

The greater power includes the lesser

Here is the argument that the "perpetual union" theory cannot survive. If thirty-eight states can lawfully abolish the entire federal government tomorrow morning by amendment, dissolve it, replace it, or restructure it beyond recognition, then it is absurd to claim that the far smaller act of one state departing is somehow constitutionally impossible. The law has a maxim for this: he who can do the greater can do the lesser. A power large enough to erase the whole federal government is obviously large enough to release one state from it. You cannot hold that the states are powerful enough to destroy the union entirely but powerless to let a single member leave.

Madison described the states as the masters of the bargain

This is not a modern invention. In Federalist No. 39, Madison explained that in ratifying the Constitution each state acted "as a sovereign body, independent of all others, and only to be bound by its own voluntary act." The states were the parties to the agreement and the authors of it. They built the federal government, and through Article V they retain the authority to remake or unmake it. The creature does not get to overrule its creators.

Article V's one protected clause proves states keep consent-based rights

Look at the single thing Article V places beyond the reach of even a supermajority. It says that "no State, without its Consent, shall be deprived of its equal Suffrage in the Senate." The Framers carved out one aspect of state sovereignty and made it unamendable without the affected state's own agreement. That tells you something important: the constitutional structure assumes states retain certain rights that cannot be stripped from them by outside vote, and that consent is the currency of the whole arrangement. A system that protects a state's equal vote even against thirty-eight other states is not a system that traps a state inside the union forever.

Which path is realistic

The amendment route is the cleanest answer to anyone who insists departure is flatly impossible, because it works entirely inside the machinery the Constitution already provides. In practice, an amendment requires thirty-eight states, which is a heavy lift. That is precisely why the Texas Nationalist Movement's path runs first through the Texas Legislature and a vote of the people of Texas under Article 1, Section 2 of the Texas Constitution, and then through negotiation. The amendment argument matters because it settles the question of whether a lawful path exists at all. It plainly does. More than one, in fact.

The bottom line

A constitutional amendment is one lawful route to Texas independence, and it dismantles the "impossible" claim by itself. If the states can lawfully abolish the federal government, they can lawfully let one state go. The states are the masters of the Constitution. The union is their creation, and the creation does not get the last word over its creators.