Texas does not have a special one country, two systems status and Hong Kong does until 2047. The fact that the communist party completely ignores this fact does not make it go away. https://en.wikipedia.org/wiki/One_country,_two_systems
I agree that the mainland government acted rashly. The forced implementation of the national security law definitely violates the spirit, if not the law here, and understandably a lot of people were alienated by this move. That said, the Basic Law does provide a provision for the implementation of such a law (Article 23), and there's ample wiggle room for the governments involved to argue that they haven't yet demolished the "one country, two systems" approach as yet.
Same for Catalonia, Hong Kong or any other.
Not saying it is a thing that should be done lightly, but also not impossible.
Countries exists for the people, not the other way round like China likes to think.
China's priority is national security, and given the track record of western liberal democracies it is not unreasonable. As I recall we had a civil war the last time states tried seceding.
I’m not American, I don’t agree with the Vietnam war and I think the civil war was an ethical difficult decision. That was 150 years ago so not really sure why it is relevant.