Normally when I think of simulation in dnd, I think of world building rather than the small scale combat stuff. Things like how do different polities interact, what trade goes on, how does a town survive, or why does it even exist, based on nearby resources. I tend to not want to get too complicated though so nowadays I use keywords based on those found in dungeon world to quickly sort that sort of stuff out. I'm likely going to expand a bit on that based on the WoW d20 game that had rules for making settlements.
I've thought about defining things using the birthright campaign setting but I'm not sure that's really going to improve my game vs the amount of work that I'd have to put in. I've also looked at the becmi rules cyclopedia and their domain rules, but the most I do with those is think about the levels of nobility available in domains to get a number of barons, counts, and dukes with the kingdom.