Just use Flixbus, you can get to Serbia for almost nothing and explore the countries you travel through from the bus windows.

 

I went to Sweden to meet some friends that way and later to Prague, it obviously is not as comfortable as going there by plane but the view was honestly worth it.

I used to ride flixbus from München to Ljubljana often when I lived there, would used to play Crusader Kings II on's :DDD

Also if I understand the question right, the main problem here is, "will EU bureaucrats be anal about me travelling through a joke country where bydlo are still dying of chinky coof"

 

both seem to have managed it well after big winter wave and big easter wave (no bajram wave? all mudslimes been long before pozzed already? could be) and I don't see any complications about that tbh

Is it longer?