This is not a problem of balance, but a bug. The monster/AI interaction is completely broken, and it is only recently the developers have began to admit it to themselves.
What you are experiencing is this:
You cannot expand because monsters destroy you if you disturb them.
AIs expand without regard for monsters. If the monster is disturbed it either does nothing, letting the AIs grow too strong, or goes after the player.
Your cities are destroyed either by monsters you have disturbed, or by monsters the AI has disturbed, or by AIs that have grown too strong.
The developers have finally (about last Saturday) started admitting there is a problem. Trust them to fix it.
The game will not be delayed. There is a good chance that they will at least mask the problem by release date.
Thus spoke Oracle Ceresa (I've been channeling her tonight, it must be my pain medication)
By the way, I am one of the people who win on highest difficulties. I do so by having a plan to deal with tough monsters as early as I can, but as the developers are removing strategies that are too effective, I'm having a harder time. But as Ceresa said, they are working on it.