I am trying to isolate my problem. I think I am getting closer to it by now. One of my problems was proportionality of actions and spells. The idea is that larger enpire should require more actions to manage. It order to prevent this, you can make each action affect the whole empire preventing the need of having more actions.
The system somewhat works and could be comparable to "Nations" board game where everything is centralised. The only issue was conquest which could make a player with a larger empire need to have more military actions to manage it. But I intend to use a system where more controlled cities gives you more options and flexibility, but not necessarily more power. It's a bit like in age of mythology where you can attack players to steal land, destroy building or get resources, but having more of these does not require to have more military action to protect them, because there is not more territory to cover.
The main issue is with magic. Let way I want a spell like fireball that I can use in battle. If I want to have centralised magic, then spells needs to affect my whole empire. This works well for enchantments, because I pay let say 1 mana to maintain and it will affect my whole empire which can be of any size.
But if I want to use fireball in a battle, I would need to pay 1 mana in each battle I cast the spell. But the number of battles I will make could change according to my empire size and the number of players. So in a 5 player game, I'll spend much more mana on battles than in a 3 player game. Then the average mana spent is not the same which unbalance the game.
The only solution I could see is that all spells needs to be enchantments, which is quite boring and limit the kind of spells I can make. Enchantments gives powerup but they are not as fun as fireballing your opponent as a direct attack when you need it.
So I thought of an alternative, what if a spell are activated for the whole turn. So for example, if I cast fireball once and pay my 1 mana for it. Then all battles I participate this turn get's a free fireball spells, because that spell is activated. The advantage over enchantments is that the activated spells can change from a turn to another instead of beign permanent for each turn. But their effects are both proportional to the empire.
That is the best thing I could think of so far, do you have any suggestion to solve that scaling problem?
It will create the same problem at the end, it's just the catculations that are different.
I thought of powering up spells in various method. Upgrades would change the scale of the effect: Enchant a unit, enchant all units.
But I cannot use such method since I need to give the same scale to everything like explained in the first post.
The only thing I can do is have permanent enchantment (that requires a ritual to cast) that has the drawback of beign unflexible, but that has other advantages, like beign less expensive to maintain for example.