I doubt Texas is going to secede from the US any time soon. And I honestly wouldn't care either way if it did or didn't. But, I don't know if some of your arguments necessarily work.
> Pragmatically, separating from the rest of the United States would be devastating to our economy. Much like the United Kingdom from the EU, but on a much larger scale, because the vast majority of our trade, people, and connectedness come from being part of the U.S.
If Texas seceeded from the United States, it could always do so as part of a customs union, or even single market, with the United States. That assumes the US of course that agreed to that (but it would be in the economic best interests of the US to agree). It would take away Texas' ability to run an independent trade policy, and Texas would basically have to accept various policies being dictated to it by the much more powerful and bigger US. But, there is nothing in principle impossible about such an arrangement. Indeed, the UK could end up with such an arrangement with the EU ("Norway plus customs union") – the latest Brexit deal doesn't have such an arrangement, but that is not because the EU is opposed, it is because the Johnson government is opposed. (If Corbyn was PM, it might actually happen.)
Likewise, freedom of movement of people and absence of border controls could be maintained if Texas and the US agreed. See the Schengen agreement, or the British-Irish Common Travel Area, for existing examples.
> Metro areas like Killeen and a good chunk of San Antonio would simply cease to exist if the military left.
If Texas seceeded from the US, it would need its own military. Given it has roughly 8% of US population, why could it not claim roughly 8% of the US military's assets and personnel? Obviously, the US would have to agree to that; but, it seems plausible to me that in a peaceful and legal secession, the US and Texas governments would sit down and negotiate a transfer of some US federal assets and personnel to Texas, including sections of the US military. Probably, any individual service member who had personal connections with Texas would be given the choice between transferring to Texas or remaining in the US. It wouldn't necessarily be exactly 8%, but it probably wouldn't be 0%. (Obviously certain capabilities, such as nuclear weapons, would be entirely excluded.)
> (Never mind that a lot of that oil that Texans are so proud of us dragging out of the ground has to go on pipelines that cross the U.S.)
International oil and gas pipelines are not unheard of.