If we get married in Germany, could I enter the US on an ESTA or my existing B1/2 visa and simply file the I-485 for adjustment of status?

Eligibility to enter the U.S. with a visitor's visa or through the Visa Waiver Program (facilitated with ESTA) requires "nonimmigrant intent," i.e. an intention to enter the U.S. only temporarily and then depart. Entering the U.S. in those ways while having "immigrant intent," i.e. an intention to remain permanently, is deemed to constitute visa fraud, with very harsh immigration-related consequences. Starting earlier this month, immigration authorities announced an increased focus on scrutinizing for this type of visa fraud and viewing the filing of an immigration application soon after entering, or even becoming married near the date of entry, to constitute evidence of visa fraud. Assuming that your fiance is an American citizen, options for you to be able to enter the U.S. and complete steps to become a Lawful Permanent Resident (to get a "Green Card') including (1) applying for a K-1 fiance visa, marrying within the required time, and applying for adjustment of status, or (2) marrying abroad and pursuing consular processing. Especially since errors in the application process or failure to provide a full set of necessary supporting documents can create significant delay, it would be wise for you and your fiance to consult with an immigration attorney who, after learning all of the relevant details about you, your fiance, your immigration-related history, etc., would be able to advise you of eligibilities, options and strategies to attain your goals, and who then could offer legal representation in the often complex application process.