You assume America would just casually let that happen. It’s one thing to leave a multinational body, that doesn’t cost you citizens and can be (incorrectly) argued that it would benefit The UK. There would be no benefit to America if they let Texas secede so they won’t allow it. So if America won’t allow it then Texas would have to secede by force

I guarantee you the moment a drone strike hits a Houston suburb a whole bunch of wealthy Republicans will realize playing revolutionary isn’t nearly as fun as it sounds. Shortly after most of the people in the state would gladly roll over to come back to America