And to post something a bit more constructive as well. What about adding Corfu first instead of Zakynthos? If you are on Paros, Mykonos or any other Cycladic island with an airport it would be an easy and probably fast flight connection on Aegean/Olympic Paros-Athens-Corfu.
There are ferries from Corfu to beautiful Saranda in Albania: https://www.finikas-lines.com/lines
From there, you can easily go by bus to Tirana, then by bus to Montenegro, take the train to Belgrade. From there you could again go by bus to Bosnia (Sarajevo), where you might find a rental car outfit which will allow you to also go into Croatia, as long as you return the car back in Sarajevo. It's easy to create a nice road trip loop Bosnia-Croatia-Bosnia without backtracking! End the trip in Sarajevo.