I am excited too and especially that you actually know her.
Guess my main encouragement would be that she go to the Main Archival place for Gelderland and/or South Holland. The places of import to search would be records for tiny Beesd and Leerdam. Some in the past thought Pieter had roots in Leerdam.
Leerdam and Beesd are only 13 minutes apart by car and on the same river for early travel.
Bearing in mind that our Pieter was quite well to do by witness of the fact that he brought and paid for 2 servants to come with his family of wife and 4 children, surely there will be mention of a Pieter Marcelis/Marselis in historical type records for taxes, estates etc and especially Dutch church registrations.
One thing I have never quite understood is why Pieter's wife's name is not mentioned as Dutch wives kept their maiden names forever. I would not be surprised if she were from a "Jan" family.