Nothing that drastic has happened. But I owe it to my children to be prepared.... So my question is this, what heritage am I?
Speaking legally, if you seek a passport on the basis of origin, this should be crucial to you, as you said yourself >
Gdańsk, Poland. My family left that region in 1884
It's a legal part. You should ask for citizenship of Poland if you can prove your ancestors came from what is now Poland. Don't bother yourself with Germany and Prussia. Focus on Gdansk.
Now, if you also seek to answer yourself what is your ethnicity, you just have to consult history. We live in an information age. It's easy. Go google Gdansk. It's on the Baltic. The oldest human-written data on it says Baltic is `Oceanus Sarmaticus` (Sarmatian ocean) or sometimes `Mare Sarmaticus` and linguistic science speaking in general, agree that Sarmatian name represent the corrupted name of Serbs (ie Slavs). See, coincidentally, even now in this very moment, Gdansk is the center of the Kashubian populace in Poland. They do still consider themselves to be Serbs. Don't be confused with that `Serbs`. Of those Slavs (ie Sarmatians) that are still Serbs, there are Lusatians, Kashubian, and Balkan Serbs. All are native populace where they live.
So, ultimately you are Kashub if we look starting point of your family to the USA, back in 1884. It's a complete story for your children if you lost any other family memory on it. Once in Poland, you can search on your own, in institutions, in Churches, etc. But, you have a valid indication of who you are, legally and ethnically. Yes, you need Poland`s citizenship.