This is something I don't understand as well. Why didn't Hong Kong choose to become an independent state when they had the chance? They could have thrived like Singapore does.
Hong Kong is still a more free society. So probably its not a good suggestion to make it worse than what it is already due to heavy handed approach to national security. Please check report in Hong Kong (52 points) [1] and Singapore (49 points) [2] on freedom.
In Hong Kong one can go for protest if its not breaking the National Security Law, in Singapore no one can protest unless approved by Police, recent case highlights it when a student send a tweet to protest when Australian Prime Minister Scott Morrison visited Singapore [3].
Hong Kong is still very good when it comes to internet and freedom of expression. Also it follows common law jurisdictions and judges are able to give judgements against the government in many instances. Also Singapore has POFMA [4], which is worse than rules in Hong Kong.
In Hong Kong anyone can form a company and manage it without having a local permanent resident or citizen as Director. In Singapore one cannot form a limited liability company easily unless has a local permanent resident or citizen as Director (for others need a special conditions and permission).
I have no love for the CCP but I think this type of caricature simplification is both wrong and not insightful. The CCP members are not some cartoon villains bent on world domination.
The British did not allowed them to choose either. The members of legislative created by the British is based on appointment not elections. The British only decided the reform the legislative after the handover sealed.
The CCP prevented the British from enacting democratic reforms and any self-determination in Hong Kong, under the thread of military invasion. This isn't on Britain, they tried decades prior.
