It feels like we're speaking different languages :) I mean a REGULAR map, where heroes start at level 1. The problem is that in 99% of cases, they suggest leveling up the hero's innate skill and combat skill.

Let me try to explain. There is a direct proportional relationship: a mixture of two "adjacent schools of magic" gives a small bonus to abilities. At the same time, opposite schools of magic create a powerful combination and a strong ability (for example, demonology, vampiric attack).
The developers decided that they needed to reduce the probability of "opposite skills" appearing, so it is not always possible to get them. But somewhere after a few levels, with a frequency of 3-4 levels, a new skill will be offered. They do not repeat. So you need patience and a long leveling process, or you have to reload the saved game again and again, waiting for them to offer what you need.
Now about the "combat" skill. The problem is deeper than it seems. Since heroes in the fourth game can be affected, the developers came up with a "compensation" in the form of this skill. And it follows the hero like a shadow. Until combat is leveled up, it will not stop appearing in the skill selection window. Or until all 5 skill slots are filled.
Barbarians are a different story. There is a special rule for them: until they change class, they will not be offered magic.