You should go to school in Germany before transferring. It won’t matter that the school doesn’t teach the “American syllabus” or even that it is “mediocre.” It’s been my experience talking to exchange students (I’ve hosted several, and met many others) that European high schools tend to be teaching at a higher level than those in the U.S., so you shouldn’t be behind when you arrive. Also, there is no such thing as the “American syllabus.” Every high school in the U.S. has a different curriculum and selection of courses than the one up the street, and students set their own goals for what they want to study in high school – selecting subjects and difficulty level.
Regarding ECs, those are never on transcripts. They only show up on university (or private high school) applications when you answer questions about them or write about them in essays. High schools don’t track your ECs, just your grades. It is up to you to track your ECs and be able to describe them as needed.
American high schools will know what to do with your German transcripts from your school showing your classes and grades – don’t worry, they have students arriving from countries all over the world, and special departments to process the transcripts and all the differences in language, grading, etc. to ensure you get credit and get enrolled in the right classes when you arrive.
It won’t matter that you attend two different high schools – many students do that even within the U.S. A university will collect transcripts from both. That is why it is important that you attend high school in Germany rather than homeschool, so universities can get your official transcripts from there if needed.
Kids who homeschool in the U.S. do so under strict requirements when they are planning to go to a university, and they usually get help with this from homeschool organizations that tell them about state laws they must follow, how to document their work, etc. It’s hard enough for these students to prepare everything to try to apply to universities – it would be much harder for you in Germany to prove your education if you homeschool there.
Yes, in general, homeschool transcripts can be “inferior” in the sense that they are more difficult to prove and compare – this is not a reflection on the amount or quality of learning however. To simplify the process, some homeschool students take courses online through “accredited” programs and they pay some tuition for this, but then they have official transcripts coming from a source that is recognized. In essence, they are also enrolled in a high school, although remotely so they do the work from home. You could always explore that option (it sounds like you have done so), but Americans tend to hold European schools in high regard, and I strongly recommend that you enroll in your local high school in Germany before transferring to the U.S. high school. I doubt anyone will make the distinction here that your German school was mediocre.
Regarding ECs, American students find the time to do them on top of high school, and you should, too. Unless your ECs are of the nature that they require significant time commitments during the day and possible travel – e.g. you’re an assistant at parliament, or you’re a superstar athlete or model or musician or actor on a grand scale – then you shouldn’t need to homeschool to do ECs.