>

I do want to point out that the Overton Window is meant to kind of neutrally represent what's considered politically acceptable in a given society overall, not what individuals agree or disagree with. So, for example, 15 years ago opposing gay marriage was objectively acceptable and within the Overton Window, and today it is not, even though of course there were strongly pro-LGBT people back then and there are strongly anti-LGBT people now.
As for where we should draw the line of tolerable vs. intolerable political views, I think your first category would be answered very broadly by most people, and there's a pretty big jump between two and three, so I hope you don't mind if I change your categories up a little bit. It's a complicated topic, but I think I would divide things like this:
Something you disagree with. Everyday political disagreements not covered by the other categories.
Something you strongly disagree with; you would still be willing to be friends with a person with this belief. For me this would be beliefs that I consider foolish, damaging, or immoral, but still compatible with a liberal democratic society. So, for example: hardcore libertarians (whether left-wing or right-wing), principled social conservatives, democratic socialists, "defund/abolish the police" types, separatists (I'm Canadian so I'm thinking of Quebec here), and so on.
Something you vehemently disagree with; you would not want to associate with a person with this belief. For me this would be beliefs I consider totally morally unacceptable and/or hostile to our liberal democratic society. For example: openly bigoted people, revolutionary socialists, religious fundamentalists, prison abolitionists, Alex Jones-level conspiracy theorists, and so on. 
Something unforgivable that may be extinguished by censorship or by force. I would reserve this category for those who openly and explicitly seek to destroy our liberal democratic society and/or advocate violence and hatred against groups of people. Neo-Nazis, fascists, white supremacists, anarchists, "solidarity with the DPRK"-level communists, Islamists and other religious extremists, and so on.
I say "may" because while I believe the government should not hesitate to bring the hammer down if these groups pose a threat, actively hunting them down may be counterproductive and hard to balance against constitutional rights; there may be a gray area between the previous category and this one. Exactly where we draw that line of course brings us to the "paradox of tolerance" and is a whole other discussion.
The system we have right now in USA is fine where everything is free game except immediate directed calls to illicit activities such as violence or disorder.
Agreeable speech does not need any protections, only disagreeable speech needs it and no one should trust a government to determine what non-violent speech is acceptable.
Let the marketplace of ideas and social pressure determine what becomes mainstream or not. Force of law shouldn't be used to shut down mere ideas. >


I understand the frustration that a lot of people have with the requirement that only incitement of imminent lawless action can be prosecuted. This makes it so that someone can say things that are likely to lead to violence as long as it would only occur at an indefinite point in the future.

It's a bad rule... until you contemplate the alternative. >

I'll talk about anything with anybody.
The worst ideas are the ones that most need drubbing in the town square.  You don't want that stuff to fester in the dark, for the same reason you don't want it held unchallenged in the light.  Discourse is the mode of social progress.
Extinguishing people to quiet dissenting opinions is the action of a propagandist.  After that precedent is set, it doesn't matter how righteous the propagandist's message was, it will turn shortly to elitism and hate. >

The thing that comes to mind immediately as what I consider outside free speech tolerance is facially untrue claims to deny elections, Ala Trump and MAGAs for 2020. Even then though it can be tolerated so long as it isn't widespread enough to actually be getting officials to overturn election results. After that violence is acceptable, but unfortunate.  There are lots of stupid ideas like that, flatearthers types that so long as it remains on the fringe society can weather them without too much trouble.