My typical MisterCaverning plan is to write up a metaplot mindmap that spans 20 levels with multiple key plot points along the way (say every 2-3 levels), and then wing it to fill gaps.
The personal setting we play in when I run is unpolished and kitchen sink-y enough for me AND the players to have plenty of bullshitting room to make up whatever we need to make those jumps between major plot points as smooth, PC or metaplot centric as the group feels like in the moment.
The plot points themselves are mapped pretty well as the group encounters them (to save me the effort of levels 8+ in most games

), zooming in with building layouts and key NPC and minion builds as well as the mini points central to that major plot point, again allowing bullshitting insofar as it meanders its way through those minipoints.
In the end the campaign is a weave of player-driven actions that I couldn't account for which become, in essence, MisterCavern-post-facto-approved "just so" narratives to link major plot points, as well as the GM-supplied plot links for when the players wanted to sit back and let me do the heavy steering for a while. It's working very well and I'm quite proud of it, and the players all seem very satisfied.