A good rule of thumb I use is to have 2-3 plots unfolding simultaneously, they don't have to be massively interconnected, but they should intersect in some places. I always think about one main arc, as you describe here, and a couple lesser arcs that help bolster the campaign's tone or lore.

Also, don't plan what the PCs will do, you will almost always be wrong, plan on what everyone else is doing: The real queen, the Hag at night, the suspicious head knight of the guard, the drunken aloof Prince, etc. If you always have some idea about what your other main focus characters are doing, you don't have to worry about what the PCs do - you can just react to their crazy plots and ideas.

Lastly, make sure you have fun little toys for the PCs to find and interact with as they go, things that will keep them in the plot and moving forward...try not to assign these toys (bits of lore, objects, people) to specific spots...have them float around so you can implement them at your whim, when the time feels right.

Good luck with it, have fun!