Would you advocate invading a seceding Texas?
It is not a matter of what I do or do not advocate.
The question is: Do you think the Federal government would stand idly by while the second largest state in the nation attempted to erect international borders between itself and bordering states, raise an army, seize (or attempt to) seize Federal property, and do all the other things an independent nation would attempt to do?
As noted in my original post, if this very unlikely event was ever to be attempted, it would be an existential question for the Federal government. Attempted secession is an event it has faced and successfully answered before.
As for invasion, that implies a substantial degree of separation would be permitted to occur prior to decisive Federal action. I personally don’t think it would ever get that far. I think the Federal government would allow sufficient actions to occur to support successful prosecutions and then swoop in to make enough arrests to extinguish the movement. If resisted aggressively, Federal armed force sufficient to overwhelm that resistance would be deployed and utilized.