I know exactly what you mean. During playtests I experienced this too, but sadly, just like you stated, there is little to be done about the AI. I tried to make the mission more atmospheric, but was limited by massive framedrops if too many objects (like destroyed tanks, bodies, craters etc) or particle effects (like smoke) were added, so I tried to add as many as I could on the 'main route' the player most likely takes. The map is a bit large for a large amount of objects and such, since they kinda must be somewhat scattered about to make it look convincing. I believe the map is also contributing to this feeling with its massively wide and empty streets.

Now, I wanted to make the players fight for every meter, going through ruins and rubble to fight the desperate defenders, but when I tried this, the AI simply would either just stand there or kill the player in an instant. It just wouldn't be an enjoyable experience, so I had to scrap that idea. A bit of that idea is still present with the house clearing task, which is also borderline unenjoyable, depending entirely on whether the AI has decided to just stand there and stare at a wall or actually shoot at the players (and kill them in an instant).

It's really nice of that you took the time and analyzed the AI and atmosphere. Feedback is always helpful, since it lets me know what works and what doesn't. I appriciate it