Development of a Necropolis mage hero

By the way, regarding "education," I understand what you meant. Does experience accumulate faster with it? Does that mean the hero's level also increases faster? That's important. But it still requires some thought.

Yes, although it's not necessarily essential. You either take it at the beginning or don't take it at all.
It's not essential because, with a decent game pace, you'll gain the necessary levels anyway.

I didn't quite understand the part about elementals. Could you explain?

1 summon - 1 turn for the hero, during which you can't do anything else.
Teleport/dispel/drain/stone skin/ice armor/regeneration/time stop/petrification/hypnosis, etc.
Against the AI, 1, maximum 2 elementals are enough; after that, it's just for fun. Against a human player, I think more than 2 elementals on the battlefield will only indicate your tactical mistakes - you didn't heal, didn't remove a debuff, didn't take someone out of the fight.
After all, you don't necessarily have to kill the elementals - you need to kill the main army. For blocking ranged units or removing retaliations, 1-2 are more than enough.
I personally prefer only the Dark elementals for the Undead faction; they remove buffs from the opponent and have no retaliation, or, as a last resort, Earth elementals - they can disable or save with petrification.
In terms of the overall usefulness of elementals, you can also add Light elementals (light elementals) due to their multi-purpose retaliation-healing ability.