So atm most of the cards simply passivley enhance the performance of your fleets in a non interactive way. Meaning that there is no real cat and mouse game between the utilized card strategy of you and your enemy.


I suggest cards instead of giving bonuses to this or that would effect the behaviourof ships and weapons and the positioning of ships:

Examples:

A card that forces say corvettes to retreat from battle at the mid point.

A card that increases the chances of ships to be organized in battle a certain way.

A card that makes a weapon behave differentely.

A card that changes how lanes are positioned or how ships are dispersed on the lanes.

All these can be % based to allow in some mild RNG. 


The point is that cards do not enhance your fleet in simple ways but their usefullness depends on your planning and on what cards your enemy also utilizes. 

Card usage should not be limited to one card and could possibly increase based on tech or some event. You pick say 3 cards to use and then for a certain amount of time you are locked to these cards. Every X turns or every X battles you can change one card. 

This is to allow every player to roughly assess what the enemy might do(if they have the proper intel and experience in battle with said civilization or good diplomatic relations in the past) but never be able to fully tell.

This can potentially be tied to the future espionage mechanics where a spy or spy mechanic could tell you about 1 or 2 of the cards the enemy will use or an esionage act that allows you to leak wrong info about card usage to the enemy.


Potential issues:


1)The engine might not be flexible in the right way to implement such things.

2)The cards need to be setup in a way for the AI to have a chace to understand when to use which cards based on the player's chosen cards.


TLDR

Make cards change fleet and weapon behaviours and positioning as well as battle stages instead of simply giving you flat out bonuses.