Hi, I am an American living in Bucharest, Romania so I am pre-conditioned to having to drive long distances to reach something. I have driven several times from Bucharest to Budapest, and have driven twice from Bucharest to Dusseldorf, Germany. Actually just got back last week.

I imagine it's a lot of small towns and little to no highways in Croatia, Bosnia and Serbia. That would surely not be your fastest route. My trip from Dusseldorf to Bucharest was something around 2100km in total, and I usually stop at the Hungarian Romanian border for a few hours. The driving in Romania is too stressful to continue.

I crossed from Germany into Austria at Passau, then to Vienna, Budapest, Szeged, Nadlac, Arad, Sibiu and then take the Olt Valley to Pitesti to pick up the Pitesti-Bucharest highway. There are two new highways opened, one that goes around Sibiu which is 16km, and then another near Deva that is around 50km. If you time your arrival in Romaina on a weekend, there are no trucks permitted through the Olt Valley and it's smooth sailing. I usually cruise at 120kmh through the pass, slowing down for the towns and then opening it up again. Police are usually only found in towns, and most of the time they never try to stop a motorcycle. You would have a nice view in the mountains of Romania while maintaining a good pace.

Arad to Bucharest, ~600km is about 6 hours of driving by car, and that's pushing it. The 800km from Bucharest to Budapest is 10-11 hours by car without stopping, not even for a toilet. In Romania you have to be aggressive. Bulgaria, that is something else. I would obey the speed limits or carry some bank notes with you.