close off their borders to preserve their identity
If your ethnicity is your identity, you're by definition a xenophobic bigot.
since I belive in the self-determination of peoples.
Except for people who want to move to the ethnostates you support, of course 🙄
Trying to violently establish an ethnostate is of course something I cannot agree with.
You literally can't establish and maintain an ethnostate without the use of a lot of violence and oppression against people of other ethnicities. That's just how the world works.