Heroes of sword or magic?

With a fully developed Strength skill tree, the warrior gains immunity to magic and excellent ranged attack. With this setup, an enemy mage hero can be defeated in one turn; all that remains is to deal with their army.

That's not the point. The warrior excels, especially in the early and mid-game, thanks to a fully developed Warfare skill tree. While the armies are still small. Let's say, with 2-4 creatures, he can easily win, but with 5-8, problems will arise. And the further you go, the more troops there are, and the harder it becomes for the warrior. But the mage, on the other hand, will be able to manipulate these large armies, and the warrior will have a tough time.
There is only one ambiguous point here – the tactician hero. He is useful for both, unless we are talking about a very fast-paced game.