Are you arguing that there is neither America, Canada nor Australia but only British Empire?
By America we will mean the USA. Don't mix the concepts of country, nation, and state. There is no nation of USians, Canadians, nor Australians. The state of the USA is a breakaway state from essentially all of the Western European empires (except Portugal I guess). Canada and Australia are still part of the Commonwealth. If that's not neocolonialism, I don't know what is. Literally Canada and Australia still recognize the British Crown. Canada, the US, and Australia are full of European national people occupying the territory of and displacing the indigenous nations of those countries.
They have never seceded from China.
Because the PRC said that the moment Taiwan does so, there will be war. Taiwan is forced by the PRC to keep the ‘China’ label.
They have had 70 years to secede. They spent 42 of those years murdering everyone who talked about secession, both in Taiwan province and abroad. The PRC didn't force them to do that. That's what the Han Chinese local government of the province chose of their own free will.
But still, no other country in the world would tolerate a secessionary movement either. The US wouldn't accept California seceding. Canada wouldn't accept Quebec seceding. Mexico didn't accept Texas seceding. Secession is always met with violence. Doubly so if the territory that is seceding is currently occupied by a geopolitical rival military that spends years talking openly about building a Pacific Kill Chain, Kill Web, and Hellscape infrastructure in the territory, as the US does regarding Taiwan.
How is it of relevance how Quebec handles its affairs? Canada is exerting control over Quebec. The PRC never did over Taiwan. Not now, not ten, not 50 years ago. Never.
This is a favorite line of the imperialist propaganda. The PRC is a government of the nation-state of China. China has included Taiwan as a province for over 400 years, even when it was illegitimately occupied by Japan. Ming China and Qing China are just China. The PRC is just China. China is a continuous concept that is not determined by which government it has.
In pretty much every context you can think of, a secession would cause a civil war.
There was civil war
Yes, but not because of secession, as we've established pretty thoroughly.
the PRC tried to conquer Taiwan but failed
This is more equivocation and weasel language to advance imperialist propaganda. The PRC did not try to conquer Taiwan. The PRC attempted to force the KMT to surrender. The KMT were occupying the Chinese province of Taiwan, much like the Texas National Guard and the US National Guard had a stand off, the US National Guard was not trying to conquer Texas, it was attempting to enforce the law and get the Texas National Guard to stand down.
And it's very convenient to tell the story that the PLA failed to get the KMT to surrender while conveniently leaving out that the US turned the KMT into a protectorate and proxy 6 months after the PLA won civil war. Do you really think the KMT would have been able to even maintain military readiness as an isolated island? Absolutely not. The PLA's failure was in not being able to get the KMT to surrender before the Americans decided they wanted a proxy in the region to threaten China with.
So there you go. De facto, Taiwan is not part of the PRC
Again, things are part of the PRC, they are part of China. The PRC is the government of China. The parts of the PRC are the parts of the government. The parts of China are the parts of the nation-state, and Taiwan is very much part of the nation-state of China, while not being part of the government of China. These are very distinct things. And the reason that Taiwan is not part of the government of China is literally the force of illegitimate military intervention by an imperialist super power whose goal is the subjugation of China, and who's mechanism is to carve off territory from China and use it for military violence.
De facto, Taiwan doesn’t even want to be China anymore
Well, the Han Chinese in Taiwan don't really get to choose to not be Chinese anymore than Washington could choose not be English. If you advocate for relocating the Han Chinese in Taiwan to the mainland and establishing a state for the Austronesian nations to self-govern, that would be legitimate. But that's not what's happening. Instead, we have a genocidal military imperialist spending billions on creating a political crisis in a country on the other side of the world to advance its global spanning imperialist ambitions while it maintains the cultural genocide and occupation of many nations.
Austrians are of German origin. Are you arguing there is no Austria but only Germany?
There are three distinct nations in Austria - Germans, Slovenian Slavs, and Croat Slavs. The Germans in Austria became Austrians by the military force of Western imperialists (France, UK, and US). There never would have been distinct identity of "Austrian" without it being forced on them. The Germans in Austria even tried to resist the forced split. So, what we have in Austria is another example of imperialism forcing people to adopt new identities in order to express imperialist dominance over them.
What about the Swiss? Should they be divided by Germany, France and Italy? Belgium
The Swiss proactively chose to form a new nation, not a new state, a new nation, over 700 years ago. Nation, the word, refers to the concept and context of birth. Belgium is the opposite. It is a bi-national state of French and Dutch people. They are incredibly distinct people and they have not merged their nations through a multi-generational process. The French, German, and Italian tribes living in Switzerland physically separated themselves from their nations of birth and then mutually agreed to integrate their tribes into a single tribe, and included everything in that choice from language to cuisine to marriage... everything became a subject of mutual agreement for the establishment of a distinct nation. Had a group if Italians just decided to do this as Italians, they would just be Italians, but by integrating multiple nations, the Swiss created a new nation. This is how nations have come about throughout human history and why we aren't all just part of the "human nation".
are you therefore arguing that the PRC should piss off from the regions they’ve annexed in the past and where they enforce Han culture since? Tibet, Shinjang? All the other parts of the PRC where the Han colonised?
Chinese colonization of the region is necessarily a distinct process from European colonization. The European concept of nation-states emerged from their distinct geography which was shot through with many many physical barriers that kept people isolated. This is why Belgium and Austria are different cases than Switzerland. Belgium and Austria are geographically contiguous with their historical forebears and the borders are politically drawn, whereas Switzerland was a physically isolated territory that could hold people as they became a nation.
The geography of China is essentially a huge flat bowl surrounded by impassable barriers. In this context, warring tribes could cross the entire region limited only by their momentum. In Europe, there were natural stopping points where tribes would dominate within their physical boundaries, and thus war-like cultures were a way of maintaining their distinct identities. In the Chinese contexts war was universally devastating.
Thus, the two phenomena developed entirely differently. European culture could afford to be as violent and belligerent as they could be because nature naturally stopped them. Chinese culture discovered that runaway violence would ultimately kill everyone and they needed a different solution - The Civilizational State. Under this model, the regions maintained their own identities, but not their own localized "monopolies" on force. The concept is contradictory in this context. In Switzerland, a monopoly on force means within the natural borders of the Alps. In China, a monopoly on force within multiple groups in the region is just not a monopoly on force but rather regional warlordism.
This is why Tibet is historically a protectorate of China and not a colony or an occupied region. The Chinese state recognizes the existence, history, validity, and culture of Tibet, but it will not permit Tibet to defend itself any longer. Once gunpowder entered the combat theater, Tibet was completely unable to defend itself. In the age of industrial war, Tibet remains completely unable to defend itself. China recognizes that this means the Tibetan territory will be protected by someone other than the Tibetans. The choices are the 600-year genocidal white supremacist monocultural Western Empire, or the Chinese civilizational state. The Chinese state could be just as genocidal and Han supremacist as the Western Empire, but it's not.
History has demonstrated that this system has served all of the people in the region comparatively well at this point. In Western Imperial holdings, languages disappear, children are tortured and killed for speaking it, laws are created to criminalize being part of the occupied community, and settlers rape and murder without any accountability. Comparatively, Tibet, Xinjiang, and the other autonomous regions within China still maintain their culture, their language, their religion, their traditional practices, their history, their identity, etc. There are certainly tensions, and it would be much better if these tensions were not there and could be resolved more quickly, but unfortunately these tensions exist within the larger context of Western white supremacist genocidal empire and difficult choices are being made daily by everyone who is working to assert sovereignty against that empire and potentially aspire to a day when that empire falls.