Ah thanks for the questions
Not sure about the axes but I sent a message to a friend who knows everything about the Germanic tribes and will reply here once he's informed me.
Pontic chariot archers- I think they used a few chariots but I've never heard of anything other than the hellenistic Scythed chariots like the Seleucid ones, who were simply used to break up or demoralize enemy lines. Apart from the Celtic chariots, who might have been used by the Galatians in Asia Minor, too, but they rarely used archers and even if there was a missile unit (more likely javelin throwers or slingers) on the chariot, it would not have been their primary equipment.
The Onagers have the same problem as stuff like the Harii- we know it for sure for the time of the Principate, but less so for the Republic. There is evidence that they were used, but not too often. Maybe they should only be recruitable at a very late building?