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

It's difficult for mages to get good logistics. Around level 20+, you might find a Golem March spell, but I don't think it will appear earlier. If you're playing against the Necropolis, I would recommend taking light magic. With Righteous Fury and Blessing. Otherwise, the Necropolis really likes dark magic. They also have summoning spells, so you could learn Banish. And also: since they are powerful mages, why not take Jaliba for an experiment?