Improving windsurfing skills is a slow progression. Your time on the water and willingness to push your limits will determing how fast you progress. I understand where you are coming from and in time, you will likely have at least 3-5 sails in your quiver.
The jump from a 5.9 to an 8.5 is a BIG jump in size. Finding the 8.5 too big at this point is understandable. The next size up from a 5.9 should be somewhere between a 6.5 and 7.5. Usually, a one meter jump in size is about right with the mid range sails. If I were you, I would look for a 7.0 to fill the gap. When you get used to the 7.0, then start on the 8.5.
One day you may want to work up to a 9 or 10 meter sail, but don't rush it. You will know when it's time.
Soon you will probably want move up to a higher performance board without a dagger board, possibly the Futura 122, 133 or 144. Once you get your Rio up on plane (dagger board all the way up) and are moving along at 20 + knots of board speed, you will understand what it's all about.
Regardless, your next purchase should be a 6.8 - 7.5 sail. Also, if you aren't yet using a harness, you should buy one and start getting used to it. This alone will allow you to progress much faster because you will be able to spend a lot more time on the water.