and nice to hear of you. For find out the right birthdate of your ancestor Willem Muller I want to give you the e-mail of the catholic Erzbistum in Mainz who shares the dates of Dromersheim. email@example.com
I think his name is write down Wilhelm Müller in the Dromersheim archive of the office in Mainz,you can write the surname also Mueller. It is the same like Müller.
They work for a fee but this problem you would have at each office in Germany and very often the offices need time to help you. I know of several people from months they had to wait. Good luck.