It's not activism that most ex JWs provide, it's support, encouragement and exposure.
It's also not what most current or exiting JWs need - again, that would be support and encouragement.
For instance, I gather Simon wouldn't consider himself an activist
Correct. I think one key part is that I have no desire to "make" people leave the WTS. Why would I? What right do I have to try to force someone else to follow my beliefs? If they are happy, leave them be. I remember many many years ago being in a chatroom (are they still a thing?) with some current-JW who was telling me how wonderful and helpful the local congregation members were helping her with her severely disabled child in an isolated community (I think it was Australia). Should I have told her "it's all lies, flee to the outback immediately!"? I didn't, because I didn't think she needed saving from anything - the local religious group was working exactly as they should in that case. Even if I had been aggressive trying to convince her it was all lies, would it have helped? Would it have accomplished anything or just made her feel attacked and maybe upset?
IMO this is why "activist" has such negative connotations. Fundamentally, for all the noise, they end up supporting the thing they claim to be against. Nothing makes me want to get in a car and drive than a climate activist (even more so if they are willing to lie in front of the car). Nothing makes me want to eat a burger more than hearing a vegetarian activist. Nothing would have made me less inclined to leave the WTS when I did than someone aggressively harassing me for being in, or ridiculing me for who I was.
Many times (not always) an "activist" is really doing things to work out their own anger, not to actively help anyone or even stop the thing they claim to be active against. Sometimes it's just an excuse for someone to be a dick.