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

yes, I shouldn't have done it that way. The manual says that you get a new Skill to develop, then a Skill to improve an existing one, and finally Abilities. Since it's level three, Jalib usually has Luck developed, so the first time, he got Magic Resistance and Spellcasting. I shouldn't have taken the Ability. I should have taken a Skill. I'll learn from my mistakes.