That’s interesting. What prevents you from just opening a Vanguard account like any other American? Friction on the German side?
Anything I make on a non-retirement account has to be reported to the German tax authorities, too - and I'll probably have to pay German taxes on those gains when I pull money out of them. Germany taxes all income earned by its residents, no matter where the source, but if my husband (German citizen) were a US resident, they would not tax what he earned in the US.
It can be difficult to achieve but the optimal situation is to be a citizen of a minimal or tax-free country, reside in a separate country and do business or work in a third.
Treating countries/governments like companies that have to vie for your business is a perspective that helps protect oneself and family.