The T4 promotions for the Ranged/Mounted Ranged lines would still be different, though, right? Would the Chariot still have access to T4 Skirmisher promotions even if its Mounted Class was removed? Would that even matter (not many Chariots will get to +100 XP pre-Skirmishers)? However, I still feel like removing Spear vulnerability is more than possible even without this (add a 50% penalty to Chariots on Spears, could be justified thematically as being being stronger against the Horses but weaker against the riders). I don't know if it's possible to just remove Chariots from the list of units that Spearmen get a bonus against, but that would be the cleanest solution.