Christine Hühn says that I was wrong and that Aschaffenburg was mainly Catholic. The books are at Diözesanarchiv Würzburg and not online.
There is some genealogy society that apparently transcribed/processed a lot of those church books from Aschaffenburg. At https://www.geschichtsverein-aschaffenburg.de/familienforschung/#81fb6c023f66b7a2b they also state a contact email address.