Why did you not want to hear about needing a centerboard or daggerboard.
For the conditions you describe, you need a longer board, with lots of "glide"
(i.e. not too wide) to get good performance.
If you learn to "rail" a Rio M, you can go upwind/stay upwind in very light conditions that would
be really slow and sloggy on a wider board or a board with no centerboard as you would have to
really rail the shortboard alot to go upwind and then it pushes lots of water making it even slower.
At 80 Kg, the Rio M would probably be better!