"Not sure what the HK protesters endgame is here." People are fed up about how the system works and about their life and the life of people they know. They want things to be "better" without having to clearly define and write a 167 page law that will probably not benefit them. People are expressing their dissatisfaction and IMHO that's a valid justification for protesters and good enough for me.
Everything you say seems correct. The problem is that movements without specific, clearly defined policy goals tend to fail. It is easy to come together to oppose some bad thing , but it is really hard to find consensus on what to do after the bad thing has been defeated. So while movements without specific policy goals can win elections, the usually cannot bring effective change. Is independence the goal here?
It seems to be developing organically as the people come to understand how much momentum they have and how much their peers agree with their frustrations.
I'd speculate that if they somehow gained enough momentum that it appeared to be within reach then independence would probably be the end goal, but who knows.