Texas could secede. The Republic of Texas joined the United States as a sovereign nation via a treaty. All parties to a treaty have the right to nullify a treaty. There is a procedure that must be followed but Texas can legally nullify the treaty which binds it to the USA. Also the USA could nullify the treaty binding Texas and Texas would then be expelled from the union. Texas, via the same treaty, has the right to divide in 5 separate states and could then sent 10 senators to DC instead of just 2.
Texas has lots of options but as you point out, with the masses of northerners flooding Texas we will most likely be a blue state in the near future. Already the once solid red Harris county (Texas most populace county) has turned blue.
Part of that is because of the influx of Hurricane Katrina folks that got here and decided to stay.
Texas did not join the Union by treaty. We were annexed by a joint resolution of Congress.
But Texas became a state through a vote in Congress, and I can't find anything in the Constitution that says a state can nullify that. Maybe another vote in Congress saying Texas was no longer a state, but I don't see that happening any time soon.