"But somehow, when you give more, it results in a very fast start."

So, can't you also tie it to the character's level?
Because in the first week, ~2-3 skeletons per battle is normal... but not in the second week.

"The logic is that Necropolis is a dead city. Essentially, a dead human settlement, if you will.
And I would like each race to have its own unique advantages, rather than one advantage for all.
For necromancers - Moander
For demons - Valafar"

The demon's idea is well-conceived.
Regarding the "dead city": it's not a city, it's a race!
Even if a Necromancer loses a city, it's not the end of the game.
The game is inhabited by creatures. The player builds a structure that suits a specific creature, and that creature settles there. Structures don't create life!
Do you realize the advantage of being able to destroy a city? The Necromancer doesn't need to kill the enemy hero (of another race) - he just needs to enter the city and wait for it to gather troops with energy, while the opponent has no way to build up his forces.
+The main specialization of humans is tied to the city. No skill from the original game disables the main specialization of the race.

If you want an advantage with a death-themed atmosphere:
if possible, let the necromancer transform the external structures of the first level into "cemeteries"... if it's not possible to change the appearance, maybe it can be done through the "harbinger of death" skill so that only undead creatures gather in them...

Even simply destroying a city is an advantage (although it's more suitable for barbarians), so that the player has to rebuild it.

"And what does mentoring have to do with it?"
As far as I can see from the skill wheel, you can't learn both skills at the same time. Mentoring excludes the super skill. And besides "education," the necromancer has other things to upgrade, and mentoring gives a lot.