being a freelancer in Germany is possible and well-known if you have multiple clients throughout the year and follow some rules.
However, being a remote worker and employee of a US corp/startup seems to be a grey zone to me because German law requires to work maximum 80% of your time for one client only.
How do you solve that problem? Is there even a legal way to do that?
I also thought about incorporating a GmbH which then creates invoices to the US company. Nevertheless, this would introduce additional costs and probably risks (e. g. when you are no longer an employee of the US corp).
I asked some tax consultants. Nobody of them really understood what I want to do and additionally, the bigger problem seems to be the German Rentenversicherung and not only the Finanzamt.
More info: I want to stay in Germany and am a German citizen. I want to be for a significant time at the company (3y+).