"Texas is a free and independent State, subject only to the Constitution of the United States, and the maintenance of our free institutions and the perpetuity of the Union depend upon the preservation of the right of local self-government, unimpaired to all the States."
"Subject only to the Constitution of the United States..." If the Constitution does not allow Texas to withdraw unilaterally then legally she cannot do so, even if the Texas Constitution specifically said she could. Which, I will point out, it does not.
We disagree about what powers Art IV Sec 3 conveys. Every clause in Sec 3 concerns the FORMATION and/or admission of States. Not a single clause concerns the secession of States.
I challenge you: Quoting the actual words of Art IV Sec 3, what clause or clauses provide the source or sources of the implication you claim exists to convey a prohibition against secession? To meet this challenge, you are not allowed to rephrase the actual wording of Sec 3 when quoting.
If there is an implication, the implication has to be sourced in the actual wording of the section.