As you correctly state, our Constitution does not specifically prevent secession and Art IV S3 C4 does show a potential template for withdrawal, peacefully, by a state, or states, by gaining the consent of the legislature of that state, or states concerned, as well as that of Congress.
However, I do not see your potential of counties option to secede. That would be, based on the potential template, up to the state itself, its Constitution and its legislature to decide, IMO. If the state did not consent to it, pursuant to the stated Art IV section and clause, it could not be done without the seceding State's legislative consent. Of course there could easily be some horse trading by the National Congress to get their consent to secede...
It might be a serious mess, but if a state truly no longer wants to be in a Union which is going in a direction that it and its people clearly do not want to follow, it might be a bigger mess to try to keep them in such a dysfunctional union. Like going to war, it should not be a impulsive or capricious decision, but if they want out, like we did from Great Britain in 1776, a nation should not have to spill blood to keep them in.
Matter of fact, I think passive resistance and non violence as practiced by Gandhi and Dr. MLKing, Jr should be the method of choice. If all the people in Texas no longer complied with Federal law, did not pay any of the various Federal taxes, refused to work in conjunction with Federal agencies... what could the Feds do?
Cant put 27 million Texans in Jail.