The problem with that is that Sixt forbids all Audis, BMWs, Mercedes, and even Volkswagens from Eastern Europe. Looking at Sixt's Vienna offerings, trying to rent an automatic would be a hit and miss as the lowest automatic class quoted is FDAR, which means it could be a BMW, Mercedes, or Volkswagen sedan...

While driving into forbidden countries would most likely invalidate Sixt's included third-party insurance, I'm wondering if my personal car insurance's rental car liability insurance could replace it? If so, the only risk I would have is driving without LDW, although this could possibly be covered by my credit card.

Sixt should really make a paid add-on to be allowed to drive FDARs and PDARs into Eastern Europe. They do offer higher-end German cars in Eastern Europe already so they could just add on a fee to make up for the added risk with cars originally rented in Western Europe.