Unlike Brexit, the first American civil war has set precedent to prevent secession. The first state to attempt secession will be labeled a word (like insurrectionist, fascist, racist, etc.) and then, like Ukraine, invaded.
If you look to the Civil war, fighting wasn't kicked off by the declaration of separation, the flashpoint was possession of federal armories in southern states.
I agree it is largely fantasy. The federal gov would never hand over bases in Texas, ect.
It is strange that the US is one of the only countries in the world to tax citizens abroad.
What if you personally object to the secession - would the US allow you to retain your citizenship and "immigrate" to Georgia?
And, what if Florida refused to recognize a class of residents as new citizens? What would happen to all the federal prisoners located in Florida when secession happened - Florida probably doesn't want to adopt a large population of incarcerated Americans, they'd probably try to give them back to America somehow - ditto for incarcerated Floridians being held outside the state - though Florida might not want them back anyways.
I think, at EOD, the US would probably offer continued US citizenship to any Florida residents who refused to accept Florida citizenship - which would likely result in Florida being a majority expat country, since, would you rather throw your personal future in with some new secessionist government or an established government that might end up eating Florida in a few weeks anyways?
And other portions of their economy are dependent on being a port (both land and sea) for goods entering and leaving the US, much of which would be redirected to other areas unless US purchasers decide to pay extra costs to import through Texas (unlikely).
A more viable attempt at secession might look like: perhaps a new Hawaiian independence party forms with massive (>80%) and sustained (>20 years) on-the-ground support, with all federal representatives coming from this party. Eventually this party might convince the federal and enough state governments to support a constitutional amendment recognizing a specific independence referendum. That's very theoretical but I hope illustrates what actual secession might look like.
Also consider that before the U.S. civil war the southern states voted against Lincoln, the northern candidate, in fact he was not even on the ballot in many of them. If the enslaved people in those states had been able to vote things would have gone differently. So we have a unique and terrible history of states seceding based on a minority's wishes.
US has anti-Federalists, though who can work to legally diminish the power of the central government.