Adhoc brainstorm meetings are very hard to manage. If you have to discuss a complex issue, it might be worth to prepare and rehearse your white board sketch before entering the room.
On its own, a white board (or a black board at school) is not very meaningful. A bunch of words and drawings out of context. For the person who sat through the meeting, the board is very meaningful. Every scribble on a specific location on the board is a visual anchor for the entire rich discussion that was held about it.
So rather than prepare a big slide deck, maybe you should prepare your white board. Where do you put what. How do you connect elements. The whiteboard gives you the perfect excuse not to make perfect drawings. Try 3, 4, 5, or even more versions until you are left with one you like.