Just look at Quebec, they actually believe their French despite not actually being from France
Most are literally Northern French. The culture they create are inherently bona-fide French-like. Genetic traits, especially ones that lead social and behavioral differences are extremely important in the creation of a culture and adapting it over time. Genetic traits matter less for mimicking a culture. There's a difference between inventing the wheel and copying the wheel. Same for culture.
The core cultural Traits of South English, Northern France, Belgium, Dutch, etc culture is extremely similar anyways as there's lots of genetic overlap between their genetic clusters.
As for Portuguese, Poles, etc. They can mimic the culture really well anyways, and for Portuguese, it only takes one generation of mixing with Northern France to be part of the Ethnic French genetic cluster ( Probably a central Frenchman of sorts).
1/2 North French 1/2 South Italian or Ashkenazim Jew results in a North Italian of sorts genetically. There's actually a good chunk of Nobel prize winner "Jews" who are mixed ethnically. Niels Bohrs being 1/2 Danish and 1/2 Ashkenazim Jew put him in the outskirts of lots of western, central and South-east european's genetic clusters. However his 3/4 Danish and 1/4 Ashkenazim Jew son Aage is part of the Ethnic german genetic cluster and won a nobel prize in physics.
Anyone not toeing the Chinese party line would be treated as second class citizens.
They can screw off to the East Coast if they are European or West Asian. Nobody genetically belongs in the Americas. It's not like Eurasia, African and Oceania where you can superimpose all the genetic clusters for Homo Sapiens, get rid of some bodies of water, and observe a very, very strong relationship between raw geography and genetic clusters. Europeans literally belong in Europe. West Asians form multiple genetic clusters around the European genetic cluster, and oh wow! They happen to be our geographic neighbours!
Multi-Ethnic Nations, where only 1 Ethnic group is actually genetically native to the raw geographic area that the nation is located in. Will always end in a political diaster in the old-world. We are too tied to our genetic homelands, the underlying logic of things will always lead to political diasters. Once you study gene flow events, you will see that none of it was a fluke. The Neolihic Anatolian Farmers moving to Europe was not a fluke. Them mixing somewhat with Western Hunter Gatherers to create early european farmers was not a fluke. The PIE people forming from Eastern European Hunter Gatherer and Caucaus Hunter-like people mixing and the corded ware culture emerging from additional neolithic anatolian farmer genes was not a fluke. The Indo-Europeans invading Europe was not a fluke. The underlying logic behind that was to create heterogenous populations from mixing with EEFs somewhat that would genetically belong in Europe once things stabilized.
As such, someone who is 1/2 English and 1/2 Polish is part of the ethnic German genetic cluster. Once you remove bodies of water, Germany is between England and Poland.
Someone who is 2/3 Scottish and 1/3 Estonian is literally the average Scandinavian native in terms of genetic clusters. If you had 10,000 2/3 Scot and 1/3 Esotinian mixes and 10,000 native Swedes. The Scot/Estonian mix would have different frequencies of genetic traits compared to Swedes, namely less blonde hair and blue eyes... Think of the Scot/Estonian mix what the Swedes could have turned out if they had different selection pressures for traits. Same people, but slightly different gene pools.
Native Americans cannot call America their genetic homeland. They genetically cluster near east Siberians. Their genetic homeland is in North-East Siberia. They chased the rising sun and ended up in the Americas.
Since nobody genetically belongs in the Americas, you can get away with Multi-Ethnic societes (but you need a dominant religion/ideology for people to work towards), but not Multi-Racial (sub-species) societies long-term. Multi-racial societies bring great dysfunction due to our innate differences, and since we are social creatures first and foremost that is extremely cruel.
Non-whites will be treated as third and fourth class.
In my West Eurasian only ethnostate, non-West Eurasians who were there before the state was formed will just be grandfathered in but be treated like second-class citizens. I just want to avoid a political diaster. If they don't want to live under West Eurasian rule then they can screw off.
Benefits to such a state is taking the heat off of Europe from West Asian migrants. They will be made to know that only Europeans and certain European/West Asian mixes can call a region of Europe their genetic homeland. Someone who is 1/2 English and 1/2 Christian Lebanese is genetically a central Italian of sorts (Tuscany and Umbria are good matches). Their ancestral homeland would be England and Lebanon. But their genetic homeland would be some regions in Central Italy. They are not mixed race people. Scientifically speaking, Europeans and West Asians are members of the same sub-species or some taxa below species if you want to be politically correct..
There would be a new secular ideology for such an ethnostate. The West Eurasian identiity would not be slop like some many modern western identities. That is what would bind everyone together, it would be a radical paradigm shift. We would also ensure that no West Eurasian genetic cluster would go extinct or lose critical mass. We can already do this with cryobanks and tactical usuage of it. I would also go a step beyond and work on de-extincting our ancient ancestors. Like Neolithic Anatolian Farmers, Western Hunter Gatherers, corded ware culture, Iranian Neolithic Farmers, caucaus hunter gatherers, etc.
We would need lots of brainpower to do pull off those god-like feats. So I would artificially select for IQ/general intelligence by getting former child prodigies, accomplished geniuses and perhaps professions with very high g factors to become gamete donors. Then I would create embryos for surrogacy with those gametes. Rinse and repeat every generation.
At this point, I'm seeking to create a caste of great inventors. With a critical mass of highly talented people, they can run an innovation ecosystem then create wealth and progress for all.
Besides, with medicial science advancements, everyone's lifespans would constantly increase, so it would defeat the purpose of eugenics. The problem with state-enforced eugenics is that it can be wielded as a political weapon while parental-choice eugenics really can't.
Also we would focus on developing West Asian nations once they drop foolish ideologies, political goals and religions. It would take a lot of heat off Europe.