Your comment -
"but not too confident using the foot straps. I find I tend to rail upwind by getting into the straps as they seem a long way from the centreline of the board and can't keep the board flat by pointing my toes. "
This makes me think that you are trying to get into the straps too soon. There is planing and then there is planing. What I mean is that there is a phase where the board rises up in the water, but still has moderated drag because it's not flat on the surface. Heavier boards like the Rio are slow to plane. If you try to get into the straps at this point, you will sink the windward side of the board.
Wait until you get going faster before trying the straps. Then get into the front strap first (it will give you a secure feeling) and it will help prevent being pulled over the front of the board if there is too much wind in the sail. Then work on getting into the back strap. You must balance most of your weight on the front foot so you can move the back foot smoothly. You might practice this on the beach with no fin in the board to get comfortable with the movement.
As Rogers says, you are making great progress. It just takes practice, practice, practice.