War has many faces and layers. War has ruined some nations and pushed progress in others. Wars were fought in different lands and by all kinds of means. Some chose their faith as their sword and shield, some have placed their bets on a new weapon to which the opponent has no answer to, and others found confidence in their wealth and numerous allies, all of which having something to gain from it.

 To emulate the grandiosity and complexity of war without overbearing the game with obtuse mechanics, I suggest the game to have an upgrade mini-tree that is unlocked when war is on the doorstep, or in preparation for conquest. It's not enough to have lots of infantry, if the city they advance towards has done gurilla warfare upgrade for their War Machine. An army might get just enough advantage in victory if their tanks at the moment are just a bit more advanced or cheaper to produce than of the opponent thanks to engineers working tirelessly to give their side a competitieve edge. Wile some might defend their lands more fiercely out of zeal or in honor of their proud ancestry, bolstered by propaganda and involvement of religious institutions, others succumb to cowardice and treason unaware of the plots, schemes and webs of corruption the enemy spies have been weaving to soften their nation in advance of invasion.

The mechanic allows to choose a doctrine and unlocks new tiers with ages, while each doctrine has several temporary upgrades that might bolster the military might of the nation in form of city production projects. Some might have permanent benefits and/or have penalties that come with them. A bigger nation, just as in real world, will have the advantage of progressing through those projects, while having higher cost to them. Each new era allows the nation to adapt and wage wars by different means and under different banners, and as humanity learns to coexist it also learns to cross lines in conflicts. Each doctrine has a number of attrocities and war crimes that come at a heavy price and might not be worthwhile in a long run, and the player will have to ask themselves if they'd go that far to ensure their victory or they might have to deal with an enemy who chose to play dirty.

Some nations fight in style and have unique projects that further strengthen their might in certain aspects unique to them, other nations might look for ways to convert their cultural and economic influence into stopping power against unreasonable warmonger getting strong too close to their borders.

Doctrines become obsolete with time and if a nation chose to use faith to fuel their War Machine ages ago, doesn't mean they have to commit to it, since each new war brings new opportunities and relevant realities and new enemy might require a different approach. Some doctrines and upgrade paths are good for the long haul, while others are meant for a quick and decisive victory, but are unsustainable in the long run and might even lead to uprisings and all kinds of trouble back at home if the war takes to long.

I won't go much further in detail, I believe such mechanic will make war in Humankind unique, interesting and realistic, while keeping the gameplay tidy and elegant and providing an area for balance tweaks for the devs and additional role playing and strategic opporunities for players.

There is a Military branch of the Civic tree that covers this. It probably isn't as deep as what you are hoping for here, but it may be expanded upon to give a similar effect. An early civic allows you to choose to either make all of your units cheaper or more powerful. This covers pretty much all of the publicly released information on how civics work.

Yeah, Civics are more general and permanent, and they're earned with Civic points, while what I think here is more like projects for cities to work on to aid in a specific war effort and making wars more interesting and distinct as a playstyle.

I think this would help solve the fact that I feel the combat in this gamee from what I've played is as simple as it was in endless legend. I would love some more complex ways of waging warfare in this game than just rock paper scissors style units and terrain. I don't feel like its a major focus for the game, and I'm fine with that, I just want it to have more strategy