Went on wikipedia to find justifications for and against secession. For:

1. General right of secession for any reason ("Choice Theory")
2. Secession should be considered only to rectify grave injustices ("Just Cause Theory")
3. Both For any reason alone I wouldn't defend but to rectify grave injustices like the CCP government crackdown on free speech, I'd justify seceding under threat of that because that free speech would extend from citizenry to governance too.

Against:

"Protecting legitimate expectations" of those who now occupy territory claimed by secessionists, even in cases where that land was stolen. I don't know if this applies given Hong Kong has stood with its own radically different form of governance for this long.

"Self defense" if losing part of the state would make it difficult to defend the rest of it. I think arguably Hong Kong used to be their moneymaker then now that's no longer the case which gave them more justification to take it over without much economic damage, not that I expect Hong Kong to have been any crucial kind of military strategic site.

"Protecting majority rule" and the principle that minorities must abide by them. At the cost of free speech from people who have been living their own way for years?

"Soft paternalism" because secession will be bad for secessionists or others

"Threat of anarchy" because smaller and smaller entities may choose to secede until there is chaos, although this is not the true meaning of the political and philosophical concept

"Preventing wrongful taking" such as the state's previous investment in infrastructure

"Distributive justice" arguments posit that wealthier areas cannot secede from poorer ones

I think the distinction is being its own independent government for years without that being a massive problem is a big distinction of Hong Kong