From Budapest east to the border with Romania is quite good because you have quite a few different routes.You enter into Romania towards Timosoara..The border roads are busy..You can get to Severin on the Danube from there but you must avoid the main roads..All the heavy truck traffic from Turkey ,Bulgaria and Romania are on these roads..They don't go through Serbia .The main roads are wicked.
From Severin ride down to Calafat and cross over into Bulgaria at Vidin and then follow the Danube to Lom and Orjahovo and then south to Pleven ..Pleven to Stara Zagora..From there to the turkisk border at Svilengrad and on a busy road to Edirne..Kirklareli,Vize ,Saray ,Cerkezkoy..I would go on the train from there.If you must ride into Istanbul come from the north on a Sunday at 4am in the morning ..!!