Meditation? I need to think about it. It's not bad, though. But I always had at least 500 mana before the start of real battles.

Meditation not only adds mana (and in percentage terms), but also significantly increases magic power.

Aggressor is a passive combat skill; mages don't really need it.

It gives a small bonus and costs a skill point. That is, you'll end up with an underdeveloped warrior (still not enough) and an underdeveloped mage (because you're not increasing magic power).
We're not considering artifacts. You can't rely on them initially; you only look at the situation during the game. And even so, the more magic power a mage has, the better.

I've never used Stone Skin at all.

It will allow you to break through strong defenses earlier.
The main principle of the game is to progress, level up, and acquire key artifacts as quickly as possible.

I didn't take Tactics and I won't.

Same as with Stone Skin. If you spend three months exploring the map and break through thanks to an excess of troops, then yes, it's not necessary.

I regret not taking Water, Air, and Light Elementals. They were available to me. Then the game would have turned into a walk in the park.

This is just for fun, and not very effective, I must say.

These are conclusions only against the AI. If it's against a human, I won't argue, as I haven't played against a human yet.

It's not even about that. It's simply a matter of rationality.
Of course, due to the specifics of the playing style, you might not need Stone Skin, but skills like "Logistics," "Pathfinding," and "Tactics," in my opinion, are almost always a must-have. Although, after the fixes in patch 1.2, I somehow got completely disappointed with the game and I hardly play it anymore.