The law is vague enough so that if FBI wants, it can declare a real lot of things to be domestic terrorism. The official definition is "dangerous criminal acts intended to
intimidate or coerce a civilian population or to influence or affect government policy or conduct within the jurisdiction of the United States".
Now, what is "dangerous"? There's no objective standard to it. Some people say publicly disagreeing with the government is "dangerous" for our democracy. A lot of people thinking Twitter and Facebook not banning everybody to the right of Nanci Pelosi is "dangerous". Other people say using wrong pronouns is "dangerous". Yet other say disagreeing with CRT is "dangerous". It all depends on how the law enforcement would define it.
Next is "influence or affect government policy". If you protest the government, you obviously want to influence its policy, right? Otherwise, what's the point of it? So, as long as FBI is willing to consider you "dangerous" - and again, there's no objective definition of it, and they will suffer absolutely zero consequences if they're wrong or overzealous in it - they have lawful authority to treat you as a terrorist.
Of course, there's a serious step to overcome - "criminal act". Only the court can decide if an act is "criminal". But for the FBI is would be impossible to enforce the law if they had to wait for a court conviction to do anything - so obviously, they are given a lot of leeway in persecuting acts that they think are "criminal" - and as long as they can prove there's not a law that explicitly says what you were doing is not criminal and FBI is explicitly forbidden from prosecuting you - even if you ultimately prove your innocence they suffer zero consequences. It's called "qualified immunity", look it up. And when we're talking about "terrorism", they could do a real lot of damage to you without ever taking matters to court. The process is often the punishment, and they can ruin your life very thoroughly way before you ever see the inside of a courtroom. And we know for a fact they are willing to lie in court documents - including outright fakery - to achieve their goals. You personally probably wouldn't be big enough fish for them to need to do that - but if they need to, they will.