Timeboxer
Explorer
So as a DM, take some short cuts. Only run what you need to keep the PCs interested and busy. The rest doesn't exist until a PC looks at it.
Like you, I usually use Just-In-Time Plot Creation, with one caveat: In my experience, it's not necessarily important to even come up with your NPCs' goals ahead of time if your party is invested enough. Generally if you throw any sort of apparently random occurrence at your party, they're perfectly willing to spin up any number of possible theories as to why it occurred, at which point you can retroactively decide that what they have come up with is true, false, or true-with-a-twist. (Detail helps to make PCs curious, because they usually assume anything you say must be important; this in turn may drive skill checks to get more information to construct a working theory.)
Mixing that up with some preplanned ideas is usually a good path to approaching a sandbox-like feel, which is what you really want anyway, isn't it?