Game is really good still but I'm currently waiting for more updates before I start a new campaign. Right now the game has a few issues, the biggest is currently snowballing, after a few years it's likely a faction already rules the whole map or at least half of it. But there are some mods on the nexus that assist with that and seem to help the issue. So as for your question about owning the entire map, I've yet to see a poster on here who's in that position if I'm being honest. But I've heard rebellions are possible, however, there are more updates that are supposed to come out eventually in EA that will go into further depth with Kingdom Management, at the moment it's pretty basic.
But honestly if you want some time to kill I would consider buying it, I've got about 56 hours of gameplay since it came out, I've enjoyed it.