I don’t know why you’d think having a waiver would pacify the Texicans. Lincoln promising he had no constitutional basis to ban slavery didn’t prevent the slave states, Texas included, from feeling threatened enough to start a shooting war.
True, but he did promise to stop the spread of slavery to more states.
In any case, that was not the cause of Texan independence, it was Santa Ana.
I don’t think it’s as simple as Texas broke away from Mexico over slavery, but it was definitely an element of the matter. Mexico adopted policies specifically to encourage Anglo settlement of Texas; primarily due to longstanding American designs on the region (America had asserted ownership of land all the way to the Rio Grande as part of the Louisiana Purchase, it abandoned those claims by 1820 but still made attempts periodically to purchase the region), it was felt that the extremely sparsely populated region of Texas would be better protected for Mexican interests if it had more people. Fairly quickly Anglo settlers dwarfed the local Tejanos. Many of the Anglo settlers came from the Southern United States, and owned slaves.
I would say a very important fact is that many of these Anglo settlers from the moment they decided to move to Texas, viewed it as sort of a colonization, with ultimate goal of bringing the land to the United States. That certainly isn’t true of all of them, but there was some element of Americans looking to take the land into the union long before the Texas Revolution.
The law of 1829 promulgated by President Guerrero to abolish slavery did grant an exemption to Texas, but only for one year, there was not a general or open ended exemption. However after that one year, most Texas Anglo slave holders simply converted their slaves into indentured servitude. They would also get them to sign documents saying they owed their masters significant sums of money, and their formal pay rate was such that they were functionally slaves (they would be paid some nominal fee, but also charged for room and board, so they had been converted from chattel slaves to debt slaves.) Anglos also continued bringing enslaved people into the state on falsified indenture contracts.
The State eventually passed a law limiting employment contracts to only ten years.
One thing that kind of kept these legal changes from leading to revolution at that point was the reality that Mexico of that period was very decentralized, with extreme indifference paid toward actually enforcing the laws the Federal government passed. So in a sense many of the Anglos were just able to continue on as they had been.
The real watershed, what has often been called the “Stamp Act” of Texas history, was the Mexican law of April 6, 1830. Passed by President Bustamante, this law had several provisions essentially designed to block out additional Anglo settlement of Texas, and to encourage settlement of more Spanish speaking settlers to the region. The most objectionable part of the law, Article 11, was actually repealed after Sam Houston utilized some of his political connections with politicians in the Mexican government, but in a sense the “damage had been done.” The Anglo settlers had adopted much stronger support for resistance to the Federal government and designs of independence grew from that point forward.
When Santa Anna took over and revealed himself to be a centralizer, that was the match that the powder keg needed. The uneasy peace, maintained in essence by a Mexican Federal government policy of decentralization and lack of interest in enforcing many of its laws in Texas, gave way to Santa Anna’s centralization reforms, which brought the Texians into open war.
I would say the biggest single issue that lead to Texas rebelling was simply Santa Anna adopting centralization; because much of what had made Texas’s Anglo settlers “work” inside of Mexico was the fact that they were mostly left the hell alone. By changing that status quo it lead directly to war. But certainly at least part of the reason they wanted a hands off Federal government is because of the 1829 abolition of slavery, which the Anglos had been able to circumvent largely through Federal government inaction.