If Texas wants to seceded - provided this can be done peacefully - I say let 'em reassert their status as an independent nation.
What the Texas National Guard has they can keep (don’t want to leave 'em entirely defenseless). What belongs to the Feds gets pulled out, they can’t have that.
Any resident of Texas with a United States passport has that passport invalidated at the date of secession. They can issue their own damn passports. If that’s inconvenient too bad. If you’re a resident of Texas you become a Texas citizen on that date and are no longer a US citizen unless you apply beforehand to keep your US citizenship, but in that case you are NOT qualified for Texas citizenship - those who live there will have to decide (so I’m anticipating there will be some delay between the process to secede and the actual date it is effective).
After the new, international border is established travel across the new border will be the same as travel over any other land border of the US. Those over-staying their visas or breaking the law in the US will be deported back home and banned from returning to the US.
After that, they’re on their own in regards to policing their borders, international relations, trade, and so forth which they will have to fund solely from the money they can raise within their new nation-state.
They won’t be penniless, of course - Texas does have agriculture, industry, etc. as well as a coastline allowing them ports for international trade via the ocean - but I think they will find things much more difficult than when they were part of a global superpower. Because Texas on its own will very much NOT be a superpower. They may even find it harder to deal with Mexico. In fact, Mexico may even want them back and maybe the US won’t instantly leap to defend the place.
Likewise, aid in the event of a natural disaster may not be as forthcoming or quick to get there.
If a bunch of religious nutjobs want to turn it in to the Republic of Gilead… well, sure, but good luck keeping people inside, The US could offer asylum to those fleeing such an oppressive system and Texas will start to bleed population. And they’ll find they have little to no ability to order around the behemoth to their north.
And, since he won’t be part of the US Federal government Ted Cruz might have to get a real job. Because I doubt the New Republic of Texas will have any form of welfare.