Q: Doesn’t the Texas Constitution reserve the right of Texas to secede?
No such provision is found in the current Texas Constitution(adopted in 1876) or the terms of annexation. However, it does state (in Article 1, Section 1) that “Texas is a free and independent State, subject only to the Constitution of the United States...” (note that it does not state “...subject to the President of the United States...” or “...subject to the Congress of the United States...” or “...subject to the rest of the United States...”)
Neither the Texas Constitution, nor the Constitution of the united States, explicitly or implicitly disallows the secession of Texas (or any other “free and independent State”) from the United States. Joining the “Union” was ever and always voluntary, rendering voluntary withdrawal an equally lawful and viable option (regardless of what any self-appointed academic, media, or government “experts”—including Abraham Lincoln himself—may have ever said).