This is an amazingly ignorant viewpoint, and I'd bet the farmers in Vietnam of the 1960's and Afghanistan of the 2000's would likely care to differ. Insurgencies undermine the advantages of expensive weaponry.

And if we're talking about a civil war or insurgency within US borders: An army fighting on its own soil and against its own citizens is surrounded on all sides and can't maintain secure supply lines.

You can look up US military projections about the feasibility of engaging in guerrilla combat against even regional armed resistance in the US. The prognosis is not good (for the military). This should be obvious to anyone who's paid attention to the relatively poorly equipped VC or Taliban.

I’m curious now, how is having 1 gun per citizen not some sort of barrier to hostile takeover/occupation?

To an American, that sounds completely clueless. The soldier is likely a neighbor (who isn't going to shoot); there's no way to isolate part of the country from any other part (excellent secondary roads); they're outnumbered 1000 to 1 in any case.

Neighbours already shoot each other over politics in the US. As these things always happen, it would be a corrupt government plus a significant part of the population that supports the corrupt government.

That just leaves the last two items.

And many of the opponents are ex-military, with tactical knowledge and skills.

US military are at the mercy of their population. Think Nazi Germans with their superweapons vs punny Russians. Germans east front were utterly destroyed that it was preferably to surrender to West as fast as possible even though it came at a huge toll on Russians side. American military at most top to 1-2 millions which is only about 1-2% of their population. Whatever weapons that US military got, they can be easily commandeered by the population should it turn evil like Nazi. A lot of ex-veterans including ex-special forces are part of population who are very well verse with military tactics. Unfortunately this is uncommon in Europe or Asia side. If you reside in this places, then you pretty much have not seen some US private citizen collections that can rival 3rd world armies. Remember USA was beaten by Vietnamese. What makes you think their own citizens can't keep them in check? The parents who their children are military are part of population too. These are what most governments are scared of and ban weapons. The intention for these governments are never for the safety of their population but their own. USA people are way more smarter than most people giving credit for. They known this concept by inventing a written document that guarantees 1st and 2nd. Even UK dares not to do this even though most early Americans originate from there.

Well, many more dead (on both sides) is the point.

Grim but true. It's the nuclear option: force the government to order their soldiers to go to war against the citizenry.

Why not? It worked in Vietnam, it worked in Afghanistan.

Guns work. "Political power comes out of the barrel of a gun", said Mao.

It worked in the US, even without shooting (https://en.wikipedia.org/wiki/Bundy_standoff)

(independent of whether you think the Bundys were in the right, the fact is, armed ranchers did deter armed BLM intervention, basically indefinitely)