“...Texas could exercise the authority in the Texas Annexation Resolution and form four more states...”
You’re referring to a Texas constitution that no longer exists. Reconstruction changed all that. And even that constitution put limits on exercizing this option, I believe. In addition, the federal constitution, of which Texas is a signitory, forbids any state from being formed out of another.
That said, hit the federales repeatedly in the head with the 10th amendment until it begins to sink in.
It's not simple, but it's doable.
From Article IV, Section 3 of the US Constitution:
...no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.