Yeah that's the only reason they're allowed those guns, because any ACTUAL civil war would be decided by air superiority and armor. Honestly let's do it, it would be hilarious.
You can take out a tank with 5lb of potassium nitrate fertilizer and a few other things to make it explosive, and around 2-3lbs of brass, you can just use pipe fittings and other plumbing fittings. Melt the brass down into a flat plate, shape the potassium nitrate charge, dig down into some asphalt and cover it up, and you've got a tank buster. The molten brass from the charge will cut through even the heaviest tank armor like butter.
A plane is a little harder. I could probably do it with a few modified microwave transmission towers with the beam tightened by using a satellite dish and amplified you should be able to burn a plane right out of the sky. Really though you've got to take out the base where those planes are being launched from. A few drones with small charges on the runway and they'll have a hard time launching until the tarmac is repaired.
Guns are and can be effective. If they could manage a 30million strong fighting force that would be something to reckon with. It would be hard even with complete air superiority to take out that many armed people if they could coordinate. But that level of coordination among those that reject leadership would be difficult.
Ukraine is really showing us what a war looks like when neither side can establish complete air superiority, so it's being fought out on the ground.
IMO your best bet against fighting the Texans, if we're not using nukes, would be to offer citizenship to all immigrants and hispanic persons who are willing to sign up to fight against them and simultaneously take up diplomacy with the cartel and Mexican government (what's the difference at this point really). Then take out their infrastructure particularly any oil production in the state, and their power plants, then just cut off any hope of resupply through Mexico via diplomatic means, and cut off any resupply by sea and land from our own country and wait a year or two.
If they do succeed, I heartily recommend doing what Sherman recommended to do after the Civil War and just kill all of them in the end.
He is also talking movie techno babble, with the exception of the shape charge for the tank buster. That was actually use in Afghanistan and Iraq, but the whole using microwave towers to take down a plane.
While his theory for the plane is sound he doesn't know what equipment those towers have and there is no way to tighten the beam of a parabolic dish without changing the feed horn. Plus tightening the beam will not add power(in this case heat potential) to allow the towers to do what he wants.
It is not mental illness to recognize the threat of the confederacy or successionists. And Sherman was right. We should have massacred all white slave owners when we won the war.