Given the route over Eastern Europe you have a problem.
First of all, there are relatively cheap car rentals which are local, meaning that you are not permitted to cross borders between countries. Europe is not a single country with states like the United States; it is more like traveling from the USA to Mexico each time. So the very tantalizing offers local car companies in Germany could offer are of no use for you.
That means you have only a few options and that are international companies which are:
Now the second problem: All these companies have no problems to rent a car for a tour in the Western part of Europe, but Eastern Europe (Czech Republic, Hungary, Poland, Estonia, etc.) is a problem.
Sixt do not rent Audi, BMW, Mercedes-Benz, VW, Porsche, Aston Martin, Jeeps / Offroad
into Eastern Europe. Europcar does not specify exactly what is allowed, but you can expect resistance (outright prohibition or hefty prices). The same is the case with the other companies.
And they are much more expensive than local car rentals, so it won't be cheap anyway.
This is really a problem because people do not read the Terms and Conditions and simply book cars, not knowing that they are losing completely their insurance cover if they do not exactly state which route they want to take. And I suppose you know how expensive even a small scar on new cars can become. If your car is stolen, you must pay it completely with compensation; if you have or even cause a severe accident, then God help you. And this is for the international companies.
So be truthful and tell exactly where do you want to go. As the companies have competition, I do not think it makes sense to promote one company, simply ask for each company how the conditions are.
Trying to cross borders with local car rentals is downright criminal even if nothing happens, so DO NOT TRY TO DO THIS !