Well, in Germany you compulsorily need an MA in order to be considered for any kind of PhD Programs / Promotion. You could consider a Germanistik MA maybe in communication of science, or something that you can connect with your scientific background BUT mind that a) you must get a certification to apply for an MA, and often C1 is not enough for an MA in Germanistik, in Germany they offer specific paths for teaching, which are completely different from the literature/academic one, therefore I don't know if you could pursue at all that DAF route you have in mind, c) finally you have to consider that they do not offer any kind of scholarship or funding except for some very particular programs (and almost all universities require high tuitions & fees for international students, and oviously you will have to pay all the costs of living and housing). If you cannot be considered in the US for a direct MA/PhD you could try to start over with a BA in Germanistik in Germany, asking if they could recognize your past credits in order to spare exams and time, after this transitionary BA you could try again in the US. Another possibility is to try an MSc in the UK (or even an MPhil), they cost about 3500/5500 £ but many universities offer at least some kind of financial aid. With such a certificate it would be surely easier to get in a more comprehensive American program (in Germany they are usually very "closed" with regards to non-traditional Germanistik / Sprach-Wissenschaft paths). These are just my two cents.
PS I hold Italian BA and MA in Foreign Languages and Literatures (English and German) and during this year I considered many PhD options in Germany... until I realized that Germany would have been only a waste of time (and money) for me, because my main interest is the comprative literature Bereich (which is almost completely vanished in Germany, academically speaking). Eventually, I decided to apply to some American universities.