a more full Barbarian roster would definitely help, mid-late game are very rough on Barbarian factions. As eastern factions, Pahlava in particular, you keep getting new units until the very end.

This is an issue with everyone except the Getai i'd say, but in particular with the Lusotan. a very basic and effective roster, but after you can train Celtiberian Infantry you are kind of out on soldiers.