Actually, I think all four of these words have the same issue: none of them refer to a specific deed. Arson can be terrorism. Assault and battery can be terrorism. Mass murder can be terrorism.
Maybe we need to turn it into an adjective instead. Terror-theft, terror-torture, terror-trespassing. It seems obvious to me that those things are not the same. Yet we lump them all under one label. Maybe there's different lines where those acts cross into terrorism? It's not terror-theft unless you've stolen the nuclear briefcase? Or maybe a laptop is enough. Is that what we're arguing?
I can agree with this part. But it seems to me that the primary method by which it undermines democracy is our [over]reaction to it, like an autoimmune disorder. To be glib about it,
Terrorism: exists
Democracy: undermines self
If the goal of terrorism is to undermine democracy, reacting by shifting towards totalitarianism is self-defeating.
Well, re: “zip tie handcuffs”: https://www.snopes.com/fact-check/zip-cuffs-capitol-riots/