The large number of raider nations fully focused on war is unrealistic and unbalanced. People who build their economies cannot hope to defend against someone with 100% military improvements, when in order to improve their economy, they have to at least turn some sort of profit. This encourages a sort of parasitism in which the balance tilts towards raiders.
I offer three suggestions:
War Exhaustion: People aren't willing to bear an infinite number of children just to be drafted and die. Two things are certain in life: death and taxes, and people try to avoid both. Every 1% of the population in soldiers that are casualties in a war of aggression (started by the player, not defending), this increases war exhaustion by 1%. This decreases the population by 1%. The national average commerce reduces war exhaustion by 0.(commerce)% per turn. So a national average commerce of 28% results in a reduction from 2% war exhaustion to 1.72% (or a decrease by 3.36% per day). War exhaustion doesn't decrease while the nation is at war.
POWs: This makes recovering from war easier and makes it more difficult to raid inactives. Half of all soldier casualties from war would be POWs, and after the war, the POWs would be returned to the number of soldiers a nation has. It won't be possible to have more soldiers then barracks to avoid gaming the system. Given that at the end of a war, the loser usually has nearly zero soldiers and tanks, this helps recovering.
War Salvage: If the defending nation loses, they receive 25% of the steel of all destroyed tanks in the conflict, and 25% of the aluminum of all destroyed aircraft in the conflict.
Otherwise, the side that suffers the most losses is usually the losing side, which is usually the defending side, and only raiders benefit.