Again, when we speak about the "cultural assimilation" as game mechanics, we speak about province as a whole changing it`s culture. That didn`t happen. While province changing from let`s say 5% polish to let`s say 15% polish is pretty hefty cultural assimilation, in game terms, it is irrelevant, and
in game terms, the poles usually end up with way more polish-cultured provinces than they did IRL, if you the NF->settlement policy.
Then, polonisation is not really special, if we look at other cases of Frace, Germany, Italy, Ottomans, and Russia, they had the same proces, of assimilating the local elite faster than local pesants.
It may be more politially charged(and thus mentioned more) due to Russian-Belorussia-Ukrainean-Polish rivalries, but that is about it, really.
Russia is kinda-sorta special in that it actually did move a lot of pesants arround, so their "assimilation" of tatars actually tries to model that proces, of very low-populated provinces getting relativly large influx of population from populus parts of the country.
Well if we would have "Nobles Culture", and "Peasants Culture", instead of one culture this would solve the problem. Peasants Culture, should of course be able to change in time, but it should be lot harder than nobles culture, as it influence lot more people.
But in such situation, we should have like in every saxon provine, lausitian sorb or veleti culture as peasants culture. Some cultures, should be less resistant to culture assimilation(Lithanian, Belarussian), and some more(Rus, Polish). Imo there should be something called : "Culture strength". It should influence culture spread AND its reistance against being assimilated.
Things that should influece the "Culture strength" should be :
Religion(having religion other than their owner should increase culture strength, if they are other culture group, aspecialy if there is no country in this culture group)
Number of countries with such culture(more divided mean they have less cultural identity with a nation). If there is no country with same culture, and no country within the culture group, means they are subjugated culture. If there is country in culture group, it mean they are subjugated, but supported by the country within culture group. Unitary country owning all provinces in the culture group, should increase their strength, and country owning whole cultures in culture group, should increase this strength even more.
National Ideas of those countries, and their tech.
Time - in the meaning all those factors should change the culture strength with time - as some culture being dominant, may change, if they are long time enslaved, or former slaves might become a strong culture after ages of independence.
Also colonial cultures should be in one culture group, if there will be any colonial revolutions. Reason, is that even while they might all come from other countries, they keep same pace - they revolted from colonial powers, and became totaly new nations, with totaly new identity. I would say that USA is great example. While all people their have their roots in other countries, and sometimes even say "I am english/scot/german/french/pole/etc.", they also say "I am American"
EDIT: Also, all should start with some culture strenght level as there was already some cultures stronger than the others. (IE polish was one of the strong cultures, while German, Lithuanian, Tatar, etc. were not so strong - germans were not strong because they were divided - and because they still helt some big slav population in saxony. Tatars were nomads, and so they were like indians in americas, but just little stronger ones. Lithuanian country was a new country, and they were pagans before, so they were seen as inferior culture and so the most of lithuanian nobility who turned polish thought. Along the other strong cultures, i would point French(Unitarian country, and the fights against the english increased their patriotism), English(which is why they dominated scots and irish - again united country), Spanish, maybe chinesse and japanese). Republican countries IMO should increase the culture strenght, aspecialy if they unite whole culture, while having inferior goverment(depotic monarchy, and native govs) should decrease it greatly.