AeternusNox

Not including Pathfinder Kingmaker on this list feels positively criminal. While not *open world* as in walking around as your character everywhere, neither is Mount & Blade Bannerlord (that too is a truly exceptional game).

In Pathfinder Kingmaker you become baron of a wasteland nobody wants. You build cities, expand your territories, and rule over your domain making every single important decision.

I'd add Medieval Dynasty to the list too. While updates are slow, because it's a small studio, it's an incredible game where you go from building your own little hut in the middle of nowhere to a thriving community you fully manage. You're even responsible for the taxes for your settlement, and it goes in depth enough to have the taxes contingent on how much the lord above you likes you. You essentially get to be a bailiff.

And speaking of bailiffs, if DLC counts then the original kingdom come deliverance deserves an honourable mention, building up your own town when granted it is fun as hell in that game, again there's lots of decision making, and it's an amazing medieval sim.