As you are located in Germany, your ticket originated in Germany, and was apparently paid for by a Amex Germany, your next step is to bring what amounts to a small claims (EU monetary claim) in Germany.

Keep it simple. This is not about a voucher vs. a refund, but about the failure to pay a refund which TP agreed to pay:

1. TP cancelled and offered you a refund as an option.
2. You took that option.
3. TP provided a reference number of the refund.
4. TP, despite multiple proddings, has failed to pay the refund.

Everything else is irrelevant and will only confuse things. Forget about NEB's as they do little. Can't explain the Amex issue here, but consider a separate complaint to BaFin about Amex (although I would hold that in reserve).

Forget about US DOT. It lacks jurisdiction.