Next year I am getting my German citizenship and planning to move to the USA
I do not mean to be rude to you, Mary: does it mean that you have already migrated to Germany and now want to move elsewhere? This is against the design of migrating for citizenship. Evidently Germany must not have treated you very well.
In terms of stock and trading: in my experience in England having owned American stocks the tax is extremely high. You must understand the reasons for why international equity trading is so difficult and sometimes illegal: domestic businesses are controlled, to an extent, so that they might not be owned by foreign citizens (or if it happens to be so, the foreign national will pay absurd tax).
In terms of Germany and America I have no experience, though I would recommend to you, Mary, that you do not plant yourself in one country for the long-term (as you seem to have done with stocks in Germany) and then go elsewhere. There is a sort of grass is greener moment going on here in my opinion