First, you need to pay close attention to where you place your feet on the Rio M.
If you don't get the front foot 4-6 inches behind the mast foot and also about 4-6 inches upwind of the center line, and the heel of your rear foot on the fore/aft centerline, so your board floats nearly perfectly flat on the water, you will "influence" where the board goes by where you place your feet/weight.
So, make sure you get your feet placed so your weight is directly over the board's fore/aft centerline.
If you tip the board at all, it will turn in that direction.
Upwind rail down, board will turn upwind.
Lee rail down, board will turn downwind.
Where do you have the mast foot in the slot?
If at the back, that's where the board planes the best, but it will drive the board upwind pretty much all the time.
Move the mast foot forward progressively until you find the right balance.
The Rio is indeed a bit more "planing" oriented than whatever board(s) you probably did your RYA Level 1 on, so you may want to modify your tacking technique a bit to get faster tacks without having the board feel "nose heavy".
Be sure to rake your rig all the way back until the foot of the sail rests on the deck of the board when going into a tack.
Progressively rake the rig back and pull it in until the foot is resting at or slightly above the fore/aft centerline of the board.
This will "drive" the board up into the wind with good speed.
Instead of pulling the rig back up and stepping back (toward the front of the board to "get around" the mast foot, try this.
With the rig all the way raked back and resting the foot on the deck, STEP OVER the mast a little behind the mast foot with your front foot.
Now you will be facing the back of the board with one foot on each side of the mast, and at least one foot or more behind the mast foot.
Then move your new front foot over the mast and bring the rig back up to complete your tack.
This keeps your weight further forward on the board and this is a skill you can use later when you graduate to a shorter, smaller, narrower board.
Hope this helps,