Auto-translated
It's a good idea to weaken the Necromancer by upgrading the "Darkness" spell, making his spells cost twice as much, and weakening his already not-so-strong army with mass spells. It's not worth relying on high-level spells, as most undead are immune to them. Therefore, you can definitely add a second school of magic. It's up to you which one you prefer, but I would choose either summoning or light, as spells from these schools are guaranteed to be found in the guild, while chaos and darkness are more difficult to obtain. Also, upgrade "Education" and "Sorcery." It's quite possible to upgrade "Counterspell." I wouldn't take the 5th spell slot; it's better to upgrade more perks, although you can also develop "Luck." As for heroes, it's all standard; my choice is Orac. But you can try focusing on chaos with the hero Nazir; in that case, choose elemental gargoyles (to increase damage), also upgrade a lucky sorcerer, and of course, "Wizard's Sign."
But, as you noticed, my build doesn't include spellcasting abilities (SA). Personally, I wouldn't find it interesting to play a mage and not master any magic. Dealing with neutral creatures will be easier with magic, in my opinion. Although, of course, it's up to you, Phantom