So... should the front come to my city and I flee abroad I'll stop being a part of Ukrainian nation because I'll stop inhabiting a particular country or territory? Seriously?
How long, a decade? No. But 100 years later, your great great grandchildren? They're of Ukrainian descent sure, but that doesn't mean much more than an interesting historical footnote. If the regime that occupied it has toppled and a new regime invites your descendants back, sure. After it's faded from living memory and become myth, definitely not. The living matter, long-forgotten dead ancestors don't. And would your descendants then have the right to then return, force out the occupants of the land, and resettle? Fuck no. That's just another Holy War, and the world has had enough of those.
PS. Also can you, please, define what you mean by "ethnostate"
Whilst strictly it's a sovereign state of which citizenship is restricted to members of a particular racial or ethnic group, I'd go more for "any state which restricts it's full citizenship rights to a subset of citizens based on ethniticity/cultural lineage, or prioritises any such grouping of citizens over others in it's legal framework".
Having things like
"The right to exercise national self-determination in the State of Israel is unique to the Jewish people." is a good start. A citizen of Israel but not Jewish? Congratulations, you're legally a second-class citizen now! Hope you didn't like your right to vote, because they now have a constitutional basis to deny you it in the future.
So if Ukraine was restricting the rights of it's Russian-speaking population (which considering you have a Russian-speaking president, seems unlikely) and declared them 'secondary' to the 'ethnic Ukrainians'...well, I'd have a big problem with supporting Ukraine against Russia. There's a reason that's exactly what Putin is claiming, after all. And to my knowledge, that version of Ukraine only exists in his head.
I see that as inherently imperialistic.
Seizing a land and it's people for resources or manpower, that's imperialism.
Declaring an area of land, of-for-and-by only the people whose long dead and largely forgotten ancestors lived there, that's going against a basic tenant of Modern Western Democracy. That all those who live under a state should have equal rights to determine it's future.
People can vote for independence of their region in clear fair and open democratic referendums, they can vote for their region to be subsumed into a wider region under the same (so not whatever the fuck Russia is doing in Ukraine, obviously). Seizing it by force or kicking out all others, or subjugating the people that live there? Well, not sure what could be more imperialistic than that.