I tried to do the same thing as you and jump from a 7'6 to a 6'4 last year. In retrospect I wish I had tried a few boards in the size range between as I think I would have improved far quicker. You can only improve if you're catching waves, and going too short too fast could hinder you.
Maybe hire/demo a few boards from one of the shops in Torquay until you find something that suits. If you are progressing that fast don't waste you coin on a fresh deck as you will be looking for another board in 6 months time. Just buy second hand from Gumtree or eBay. You probably won't lose anything in resale value either if you look after them.
If you are going to go shorter, maybe look at boards that are 20/21" wide so have a bit of volume and stability.
Just my 5c worth.