I am not defending Texas or its use of slavery but you forget that the Republic of Texas was in a very weak position.
Assuming the U.S. president required a "no slavery requirement" for annexation, it had four choices: join the U.S. but give up slavery, rejoin Mexico (a very real possibility) but give up slavery, become a British possession (a real possibility), but give up slavery, or remain independent and vulnerable. Please note that President Sam Houston was very much a wild card on this as he was in 1861 when he came out against secession.
I completely understand that Texas was in between the U.S. and a hard place, but I don't think it was composed of people who were eager to align with the U.S. at the cost you describe. Freeing all the slaves in Texas would have been both financially and philosophically costly, as the economy was heavily invested in cotton farming, and to do so would have essentially rolled over on a big reason "Texicans" were willing to secede from Mexico in the first place.
And yes, Sam Houston was a wild card. 8)