I think I have found the right difficulty level for me:
(1) Very hard campaigns/medium battles - gives the enemy more resources, but keeps the battles balanced.
(2) Rome Total Realism Mod - certainly makes it harder for the Romans, my favorite faction.
(3) Restrict recruitment of non-auxiliary units to your capital. This slows down expansion and gives the AI a numbers edge.

The long campaigns are still very long, but tightly contested.