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

If there's an opportunity for long-term development, then definitely choose the Sorceress. Reasons: excellent progression path (spellcasting and education; we don't count the unnecessary Katar skill), all, ABSOLUTELY all spells cast at the maximum level are readily available, so there's no point in searching for them or leveling up magic: everything is already great! Then, get a mini-artifact for speed for the Rakshasa, equip them with Heavenly Shield, and they're ready to cut through the enemy's front lines! And then you can cast Divine Retribution, Subjugation, and Phantom. If desired, you can also hit them hard with Earth Shock. The Ogre will "recharge" quickly + it has initial skills specifically for the Sorceress: there are still 22 levels left!