But that doesn't work because Schelling points are subjective. You can't reliably convince everyone that some specific arbitrary threshold should never be crossed, because it will surely lead to doom. Some might agree, and others will see things in a different way.
That's why it's pointless to even try, and that's why it's pointless to think about freedom of speech in terms of Schelling points.
So how about just pretending there's a Schelling point at 100% of freedom of speech? Any concession from that will inevitably lead to more, and besides, losing our freedom of speech is not a clear-cut phenomenon anyway.
It's a bit like new currency entering circulation at various points in the economy - some people's perceptions of what's acceptable will shift before others', but will in turn affect the perceptions of others around them. Freedom of speech rots in arbitrary, unpredictable and uneven ways.