I have been corresponding with another researcher, and he has proved to me that I made a mistake on one of these Zentzes.
The mistake is with Peter, son of Peter. This Peter is not the one who married Catherine Siegrist. The Peter who married Catherine was actually a Sentz, whose family was from Germany. He was brother of the Nicholas Sentz who also lived in York Co PA. He also had a son Peter, who is probably the other one in York Co on the 1790 census.
My Peter son of Peter apparently remained in Baltimore Co. A 1798 record shows him owning land there. He and his father are both listed, as is Peter son of Christian.
I still see only one Peter after that, who is the one who married Catherine Shroyer. I still think he was son of Christian, because of the 1809 record. Peter Jr may have died in 1807. I currently have that as the death date of Peter Sr, but there is a Peter age 45+ in Baltimore Co on the 1810 census who must be Peter Sr.
I'll have to keep digging and hope to find some more evidence to clear this up.