if they get bogged down in enemy units they don't seem to be able to defend themselves and get slaughtered
Which is exactly what happened to them in R1. I never used chariots much in R1 and I don't know if any of the following could apply to R2:

Pick a point beyond the unit you want your chariots to attack. You want your chariots to wreak as much havoc on unit formation as possible, and move through the enemy to regroup on the other side, if possible. Chariots don't get many kills, outright, but they disrupt formations and lower morale.

Follow up the chariots with a hard-hitting cavalry unit. The unit just hit by the chariots will likely be disorganized (although if R2 "unit-blob" doesn't get fixed, it won't make much difference) and taken a morale hit. In R1, as the Seleucids, a devastating combo was scythed chariots followed by cataphracts. Only the most elite of units could take this double hit and not immediately rout....