I don't like fast-paced games; I prefer to play with full upgrades... When we play, we even agree on a non-aggression period after which we can engage in combat...

It's a personal preference for everyone, but I prefer games with an element of unpredictability. And when I talk about destroying in the shortest possible time, I mean the direct clash on the battlefield. You just need to try to defeat the enemy's troops as early as possible, especially if they have magic. An attack is perfect for this. Considering the high damage of Orcs, the benefit of additional percentages will be more noticeable than, for example, for a Knight or a Dark Elf. But defense will also reduce the already not very high damage that reaches the Orcs (rage and good defense stats are still important).

Regarding the lack of space...
Attack, of course.
Leadership.
Luck.
Defense.
Machines/Logistics (in our games, logistics and ballistae are mainly for sieges).

Yes, if you don't upgrade skills, the levels will be sufficient. But extra 3 levels invested in defense could be used for something more useful. However, sometimes there is no choice but to upgrade defense. For example, if nothing useful is offered except for it, or the level is so high that there is nothing left to upgrade except for it. If in your games the hero consistently reaches level 25+ (I was a bit hasty with 30+ :)), then defense "fits," but again, I would prefer education.

P.S. Does the Darkness skill help against mind control/berserk?

It only reduces the duration of spells. And you won't be able to cast berserk on Cyclops if the Darkness skill is more or less upgraded.