The question that comes to my mind is just how isolated these folks have been. If everyone you see 99% of the days of your life comes from a common heritage, it seems likely that many of the traditions of that heritage will be preserved (because they're universal, and thus unquestioned). If you routinely mix with people from a wide range of backgrounds (even just consuming "foreign" pop culture), that's much more likely to dilute individual cultural factors, I would guess. (But even then, pockets of cultural traditions would likely still persist in scattered celebrations or hobbies: my family's "Swedish Christmas Eve" tradition is still going very strong after well over 100 years, for example. Though come to think of it, I'd be surprised if that held up for another 200 years. When the people who remember the people who remembered the people who came from the Old Country are gone, there's probably not much of a bond remaining.)

Regardless of those details, though, I don't expect that after 300 years anyone would really think of themselves as "Central Europeans". They'd be "Centidanis" (or whatever their local label might be), and although a historian might well recognize very sizable cultural heritage from central European traditions, that old connection would only be a fact of passing personal interest to the locals. (Does anyone even visit Old Earth anymore?)