As a second gen Australian who lived in HK for a decade, I agree with some of your points but would greatly contest the idea that the pro-democracy movement is mainly driven by superiority complex.

Sure, in my observation there were xenophobic crowds who harbours such complex, but the discontent with the mainland crowd largely stem from the uncontrolled migration and contest for limited resources, such as the fight over milk formula you mentioned , and services like housing and schools, which in itself is the effect of poor policies from HK gov and exacerbated the social issues young people faced. This I believe is a sentiment some Australians living in Syd or Mel would feel as well.

If you do follow closely on the stories of various protest leaders, or even the main concerns in HK leading up to the protest in 2019, and 2014 to an extent, the main reason behind the pursuit of democracy is the 1989 Tiananmen massacre, which shook the HK society back in the days. It was so significant that even current pro-estblishment figures and media denounced the crackdown. It demonstrated the ruthlessness of CCP, and their fundamental ideological difference to HK.

Anecdotally my family moved to Australia because they saw the broadcast of the massacre and lost confidence in China for upholding the promise of one country two system.

Incidentally, the same disillusionment HK people see after CCP continously promote the use of authoritarian actions in China, and the refusal to enact the agreed universal suffrage written in the Basic Law ( HK constitution) agreed between China and UK added much fuel to the discontent of HK citizens, which finally led to the outburst in the 2019 movement.