Hopefully it never needs to get to that point because of the threat. But that’s literally what we did in the revolutionary war.

al-Qaeda + isis held off multiple nations military for almost 20 years.

Why would a civil war where half of your military isn’t blindly loyal going to be any better? Killing your friends and neighbors for the state is a lot different than killing someone you’ve never met before.

Your straw man is that it's a testosterone fantasy, and that the idea is to fight off the US Army with someone's basement guns. I'm saying the straw many is largely your representation of attitude.

Look at Al-Qaeda and ISIS. They had little more than AKs, no electronics more than cell phones, and they managed to drive us out of Iraq and Afghanistan. So let's agree that it is POSSIBLE for a weaker force to defend against a stronger force, albeit with higher loss rates, especially if the weaker force blends in with a civilian population.

However Al-Qaeda is a bad example. A much better one is the Bundy standoff from 5ish years ago. And that shows a big part of the usefulness of guns- increasing the cost of using force.
Now for the record Bundy was an asshole so don't take this as me idolizing him. But the situation is a useful example.
Put simply- there was a dispute about whether Bundy was allowed to graze cattle on some public lands. Bundy claimed a legal ancestral right, government claimed ancestral rights were removed because endangered turtles lived on the lands, but the government would still allow him to graze the cattle for a steep fee. Bundy refused to pay, so the government sent in workers to seize his cattle.
Bundy and his followers then took up armed positions to defend the cattle. The message was simple- it will take a firefight to get you the cattle. Everybody (including Bundy and his followers) knew the government would probably win, a bunch of ranchers with guns isn't going to fight off the National Guard. BUT, it would also mean a lot of blood spilled on both sides. As in, 'think twice guys, is seizing a few cows worth another Ruby Ridge type fiasco?'
Fortunately cool heads prevailed, the government backed down and agreed to bring the issue back to the courts.

The lesson remains though. A bunch of armed ranchers 'defended' against the mighty US government without ever firing a shot, simply because them being armed raised the cost of using force against them. If they'd not been armed, the government would have sent in riot cops with batons to beat them all up and arrest them and that'd have been that.

And THAT is why I say that defending against tyrants is still a valid goal of 2A. Because defending against tyranny doesn't even necessarily mean killing tyrants, sometimes it just means making oneself a harder target to tyrannize.

(And once again, I should clarify I'm not necessarily siding with Bundy. I'm just pointing out that from his POV the government was being tyrannical, and his resistance against what he saw as tyranny WAS effective.)