Alternate Caesar History

[Gaius Julius Caesar]

While I can't dispute your assessment of their perception of the physical sizes of the world, i would imagine that Caesar was possibly the only Roman who would have succeeded against the Parthians.
I doubt he was uninformed of their abilities, as he would have information from the survivors, and also there would be spies and information available to him from his local friends from the east.
He was not without resources and contacts. He was also not as easily distracted as Anthony, and also would have had him with him I imagine, as well as a good cadre of loyal officers and men.

It is a mistake to compare him to the others, because he simply was a cut above the rest, however much a person may find him disagreeable. As we all know, a flock of crows can down an eagle, but the eagle is not diminished because of this...

As to the Dacians, were they as organized in Caesars time as they were in the later period of Trajan, when the world was more aware of the dangers of the Roman military machine, and was adapting to counter it..