Which magic schools should I choose for the late game? Or should I go for a specific artifact?

The Academy's army really needs all the magic it can get!!! That's why I would choose the Stronghold. First of all, have you seen the Reactive Rakshasa? (with vampirism + speed + charge + magic shield) Take a look! It's awesome! Of course, the Stronghold doesn't have mass spells, and improving only one type is... not good overall. So, you need to adapt to a specific faction: against Humans, the Stronghold definitely rules! They have training, so a powerful Puppet Master will easily defeat, say, Paladins, and that's almost it! If we take Angels, and then Chaos, it will be useful, and even Divine Retribution!!! THAT'S IT!!! :)

Added 2 minutes later
The Stronghold is needed so that you don't fail with spells (in case they don't drop out).