That's a good overview.
> (ideally in the early stages)
That's an important part of the original COINTELPRO in the 60s. While many people know the FBI targeted MLK, a lot of the FBI's counter intelligence goals were about preventing any leader from even forming in the first place, or allowing any critical mass of black people to associate, because association is power.
> rape of possession of child pornography
Note that parallel construction probably doesn't use anything so heavy handed in most cases. "Laundering" intelligence information is easy when the target is carrying a real-time tracking device. Most of the time they probably just inform the local police when/where you are driving. Then they simply need to tail you until they see you do something they can call a traffic violation. At that point they have probable cause which is good enough to get a warrant that can be used to "find" whatever they want.
> perhaps they first send disruptive internet commenters,
We have documents from GCHQ's "JTRIG" department that explicitly state[1] that's one of their primary methods.
> JTRIG Wiki
I suggest reading Greenwald's original reports[2][3] (which contain the source documents from Snowden's archive).
> only if the target is still a threat
That's the problem - it depends on how you define "threat".
> how much do they actually do, and what effect do they have?
I doubt we will ever know for certain, but if you're seriously interested in that question, then you should listen to former Guantanamo detainee Moazzam Begg's answers. He talked[4] at 32c3 about just how far we've fallen.
Also, it's important to remember that rights are about protecting the people that society hates. When a group of people is liked by society so they aren't a target of oppression, there isn't any need for rights. It's the people that most people see as not deserving of rights that need their protection.
[1] (from [3]) https://firstlook.org/wp-uploads/sites/1/2015/06/jtrig-540x5...
[2] https://theintercept.com/2014/02/24/jtrig-manipulation/
[3] https://theintercept.com/2015/06/22/controversial-gchq-unit-...
[4] https://media.ccc.de/v/32c3-7443-the_price_of_dissent
TL;DR - Watch [4]!
edit: I know [4] largely talks about the UK. While the UK is slight ahead of this slide towards dystopia compared to the US, the US has it's own problems.