I think making it akin to Technocracy or Exalted Priesthood would be nice - leave the army damage there, as it's mostly useless anyway and one administrator (two with the highest level of capital) could be replaced with a job that gives unity, defensive armies and naval cap, like a super soldier job.
I'm wondering if it's possible to mod in a 1-on-1 relation between a pop and an army. I'm just looking at the army files. Would it be possible to disable normal army creation; but then set armies to "defensive = no" and "is_pop_spawned = yes"? I can't seem to find a define for how fast armies are pop-spawned though.
What if it gave a warrior job in every city?
I think there's enough going on here in this thread to come up with a good solution.
A major focus of 2.0, and to a lesser extend 2.2, was to reduce the number of ships in play. This made each ship more valuable and wars become more strategic. This same logic can be applied to ground warfare for similar results.
For starters, manpower is already sort of represented in the game. The number of armies of a particular species you can build is already limited by the number of pops of that species living in your empire. I don't know the exact ratio, but it is high enough that armies are easily spammable. A reduction to, say, 1 army per 5 pops, calculated separately for each species in your empire, would reduce the number of armies in play to the point that each one becomes much more important. It also makes defense armies in general, and fortress worlds in particular, much more resistant to ground assault.
Alternatively, a "Logistics" resource could be introduced to the game. This resource is a required maintenance for non-defense armies and is generated by pops in smaller amounts and by enforcer/soldier jobs in larger amounts. The army based techs could also be changed to give a bonus to Logistics on top of what they already do. This allows greater control over army composition and gives you a direct representation of how large of a ground force you can reasonably maintain. It should also be a soft cap, but going over it should impose increasing penalties to army damage/morale. Different armies can even have different maintenance costs. For example, a regular Assault Army would have a Logistics upkeep of 1 while a Slave Army would have an upkeep of 0.5.
The next issue that has to be dealt with is bombardment. I don't think defense armies generated by enforcer jobs should be safe from bombardment, but defense armies from soldier jobs should. Fortress armies were immune to bombardment before and that needs to be brought back, unless I'm mistaken and they are still immune. This would make fortress worlds actually meaningful (at least ocne the AI is completely broken), requiring great effort to invade by ground (meaning Federations, with their greater numbers, could have an easier time breaking through) or straight up cracking the planet. Pops could still be killed via bombardment though, thus ruining forts, and I'm not sure what to do about this short of making Fortresses causing 5 pops (10 for the upgraded versions) to also be immune to bombardment. Unless this happens, no amount of defense armies will ever matter.
As for the Warrior Culture civic, I think making it more like the Technocracy/Exalted Priesthood civics is a step in the right direction. Wiz himself said that the code now allows for things like resource generation to be dynamically based off of things like Barbaric Despoilers having armies on an occupied planet. Theoretically, Warrior Culture could make all armies generate 1 unity and also replace some administrator jobs with a Commander job that produces stability, defense armies, and Logistics.