How much further should you let them get?

That's the wrong question. The right question is, do you have the organized numbers to do anything productive about it?

A half dozen armed protesters are going to jail by the end of day, if not the morgue. A couple dozen might draw reinforcements, but the result will ultimately be the same. How many will you need to stand up to the inevitable military escalation? How are you going to keep that many people, who are already emotionally invested enough to show up at an armed protest, calm and orderly throughout the ordeal? What is the goal, just simple intimidation? What does "mission accomplished" look like? How does everyone know when to go home?

The kind of coordination required to accomplish your suggested outcome doesn't just spontaneously emerge from a group of armed humans. It requires extensive organization and considerable discipline. That takes time. By all means, put the effort into organization.

Thousands of untrained armed citizens gathered together in a spontaneous protest? That's gonna end really, really badly. Eventually a gun is going to go off, and then a lot of guns are going to go off, and a lot of people will die, and some of those people will be cops and it'll be hoisted as a new 9/11 and an excuse to ram through legislation that would make the Patriot Act look like your nosey aunt.

This is one of those "if you attack the king, don't miss" scenarios. Once we have enough committed people, and those people are trained and organized, we can talk about that route.