The thing about law though, is that it’s just a framework of written social contracts between rational parties agreeing to abide by the terms and consequences.
Reality is a bit different.
Texas could halt physical transport of goods/services. Refuse to buy US imports. Stop collecting tax revenue. Gun down federal employees that don’t swear Texan allegiance.
It doesn’t really matter what legal papers say, when it comes to actions.
Sure - there may be consequences for such “illegal” state actions, and the documented illegality would be articulated as official justification after administering such consequences.
But that also only matters if Texas is defeated … in the unlikely event they “win,” - they’d write their own narrative with legal justification.