Look how the Gulf states deal with citizenship and they manage to control much bigger immigrant relative to the native populations.
The only real refugees in Europe are Ukrainians.
The gulf states, really? Why am I not surprised that you point to one of the largest modern day slave states as an example. I will not grant you the assumption of ignorance. For anyone else following this conversation, ReptileMan here is pointing to this system - https://www.humanrightsresearch.org/post/modern-day-slavery-... - one widely recognized as slavery - as an example of a good way to control large immigrant populations.
Even without that, I keep my head on a swivel in any lower income areas - migrant or otherwise. It will shock you to learn this, but crime is not a phenomenon that is unique to brown people.
Which of those do you most prefer?
Would you also like an ethnic caste system, or should it be based solely on national origin, or a combination?
Or perhaps the original European model, where only non-christians may be enslaved?