If anyone is curious, the full text of the Texas Declaration of Independence, March 2, 1836, is available here along with other important documents from that time.
Of course slavery is not mentioned explicitly. Slavery was illegal. You don’t publish a cri de coeur about the government’s illegal acts while whining they are prohibiting you from committing illegal acts.
Context requires understanding the history that led up to the Declaration. Santa Ana had indeed overthrown the Mexican constitution, in the process creating federal control over the states. The Constitution of 1824 had given them far greater self-control over areas that were far distant from Mexico City.
The Declaration of the People of Texas, Nov. 7, 1835, made their discontent clear.
On November 7, 1835, the Consultation issued a “Declaration to the Public” declaring that “The people of Texas, availing themselves of their natural rights, solemnly declare that they have taken up arms in defense of their rights and liberties which were threatened by the encroachments of military despots and in defense of the Republican principles of the federal constitution of Mexico of 1824.”
What else had changed since 1824? Slavery. In 1829 Mexican President Vicente Guerrero issued a decree abolishing slavery in Mexico. Those in what is now Texas were infuriated and refused to accept it. Guerrero had to rescind the decree for Texas, but the residents there understood that federal control could be reinstated at any time.
As I said, the events in Texas in 1836 exactly paralleled the events in the South in 1861. Complaints about federal government control were explicitly or implicitly complaints about the future of slavery in their territory. Other complaints may or may not have been valid, and economic interests of many kinds were interwoven. But those were minor. Slavery was major. You cannot invert them.