Those light chariots are amazing once they get chevrons. Just need to keep an eye on them to make sure they don't get trapped at the edge of the map or get impetuous and attack at close range (which can happen).
Even at vh/vh, head hurlers cause units to rout with a few volleys. I usually keep them behind a line of chosen swordsmen. Is this worth it? I know HH have rubbish defense but would 10 HH in double line be more effective than 5 chosen swordsmen guarding 5 HH?
Slingers aren't bad but they are a double edged sword in that they cannot be placed behind your own units because of friendly fire.
Heavy chariots are tricky to use and require a lot of micromanagement. My trick is to get them to attack one unit and then immediately attack another unit to keep them moving but it's awkward to do when you need to manage other units as well. Best to use them to attack routing units or units just about to break.
In one battle, my faction leader's full stack fought four battles in succession against full stack iberian armies. His chariot unit alone killed over 800 men in one battle.
General tactics are to wear them down and eliminate cavalry with light chariots and break their infantry with HH volleys. If they don't break with that, engage with chosen swordsmen and flank with HH.
Are there units other than the chariots and HH worth looking at? Warband? Druids? Hounds? Would I be better off ignoring chosen swordsmen for HH?
On the strategic end, I decided against invading gaul (this is mundus magnus) and went for iberia instead because they have richer provinces. It's challenging but fun. The economy is rubbish but that's to be expected.