I believe that the constitution of Texas clearly states that it may remove itself from the Union if it desires to do so.
That was a carrot given to Texas to get it back into the union in the 1800’s.
I believe the jurisprudence since ratification of the Constitution has always been that reservation clauses in ratification articles have always been invalid. It's always up or down.
The idea that ratification is a "threshold event", a one-time consent that can never be undone (as opposed to "perpetual" consent, which is continually renewed -- until revoked) has always been wrong and bad, and a favorite among "locked-door/suitors of Penelope" school Yankee politicians who intended then and now to prey on the South and the other agrarian sections of the country. Big favorite of theirs, and they've enforced their view with megascale death and violence against dissenters.
The "offer" that the triumphant Yankee politicians made Texas in 1865 was that, if the Texas Militia and Confederate troops would go home, they, the new owners and Lords Ordainers of the United States, would not send William T. Sherman into Texas with > 100,000 men to burn, kill, and destroy everything in sight.
It matters not what the constitution of Texas says wrt secession. The Constitution of the United States is the Supreme Law of the Land not the constitution of Texas.
I always heard that the Treaty accepting Texas into the United States had some clause allowing the state to split into several states but am not sure about any of that.
Texas never left the Union so it could not be readmitted. The attempted rebellion, after it was put down, had no authority to do anything wrt to Texas’ participation in the US government or not.