Self determination and national liberation movements arent good or bad by themselves. In marxism, support for national liberation movements must always be subordinated to antiimperialism. Does this national liberation movement weaken or strenghten imperialism? Thats all that matters. Should we support uyghur separatists in Xinjiang or latvian separatists back when the USSR existed? Ofc not, they weaken socialism and strenghten imperialism, since China and USSR are socialist countries. Should we support corsican separatists in Corsica (an island held by France) or basque separatists in the Basque Country (a region of Spain)? Ofc yes, they weaken imperialism since France and Spain are imperialist countries. The same goes for invasions of countries. Was the Winter War (the soviet invasion of Finland in 1939) right? Yes of course, since Finland was a reactionary anticommunist state and if that invasion had fully suceeded Finland would have become socialist. We communists are internationalists, not nationalists. We only support nationalism when it strenghtens socialism, otherwise it belongs in the trash.

Now there is a counterargument to the “invading countries to spread socialism” thing, which is that if the material conditions of the country arent ready for a socialist revolution yet, the population will resist the invasion and socialism will fail. This is why China did what it did in Tibet (which you mentioned). Contrary to western propaganda, China didnt invade Tibet. Tibet became independent in 1912 after the collapse of the Qing Dinasty and the beginning of the Warlord Era. Tibet was a feudal society ruled by the buddhist monks, with the Dalai Lama at the top as an absolute monarch. Most of the population were serfs who had 0 rights and were enslaved to the monks who owned everything, including the serfs. Resistance was brutally punished. In 1949 the Kuomintang lost the Chinese Civil War and the Peoples Republic of China (PRC) was proclaimed. In 1951 the PRC negotiated the 17 Point Agreement with the Dalai Lama to peacefully reunify Tibet with China. As part of the agreement, Tibet would retain its feudal government and society.

Now why did China do this? Exactly because of the argument i mentioned. They wanted to wait until the tibetan people were ready for socialism. And this happened in 1959. Contrary to western propaganda, China didnt “annex Tibet and oppress tibetans”. What happened in 1959 was that the tibetan serfs, seeing the freedoms that their chinese brothers had but they didnt, decided to rebel against the monks. They took up arms and rebelled. The Dalai Lama was preparing to brutally suppress the uprising, but China intervened on the side of the serfs, arresting the monks and forcing the Dalai Lama into exile, where he became a CIA puppet. From then on, tibetan feudalism was abolished and Tibet became socialist like the rest of China. China waited until the tibetan people rebelled against feudalism instead of imposing socialism from outside. This is all explained in the book “When the serfs stood up in Tibet” by Anna Louise Strong, an american journalist and communist who was there in Tibet when the events happened.