Hello Daniel,
in Germany they have a mailinglist for Posen
https://list.genealogy.net/mm/listinfo/posen-lSome members speak English, but google translate would probably work too.
Other possibilities are
http://www.wbc.poznan.pl/dlibra?action=ChangeLanguageAction&language=enhttps://szukajwarchiwach.pl/I would suggest to find and search the name of the place in the polish writing , so for Schwersens/Schwersenz = Swarzędz. The polish archives often show more hits when using the polish writing, but try different variants.
Sometimes it also helps to search the names of the ancestors in different other archive for example for books or "Amtsblaetter".
Sabine