You're using Lenin's definition of imperialism. Lenin’s definition focuses on economic domination, not military or territorial control, so for the PRC’s invasion, which was "liberation," it’s better to use the modern definition of imperialism, which most people reading this will be doing anyway.

Tibet was also a serfdom, not a slave society, there is a distinct difference. Serfdom binds a person to land. Slavery treats them entirely as movable property. One is labour, one is chains. Calling it liberation is also extremely negligent and steeped in bias, the US military uses this excuse all the time, that they are liberators instead of imperialist forces.

But ultimately this all avoids the question of whether or not the Tibetan population wanted integration with China, that's the crucial part that makes it imperialist, the inability for the Tibetans to decide for themselves.

Which, again, you're making a false conflation. We've established that Europe is imperialist, yes. We've established that the US is imperialist, yes. But then you're including the PRC in an attempt to make it appear anti-imperialist -- Which it mostly wasn't. It's a very camp argument. Imperialism is imperialism, it doesn't matter who's doing it and for what reason.

Redsails is also not a good source, it's openly from an ML perspective, so it's not neutral, which you absolutely have to be when discussing history. It's also under no pretence to be academic or accurate either, Redsails is ideologically driven rather than factually driven - so it won't ever be critical of the ML perspective. You can use redsails to talk theory, absolutely, but not as a historical or factual source, it's dishonest.

China is also not entirely socialist, either, it’s state-capitalist with socialist rhetoric. They still have private property.