Can you describe the conditions you would most often be sailing in a little better?
We need the most common windspeeds in order to recommend both board and sail sizes.
I just received a new Rio M a couple of weeks ago, and it would be a really easy transition for you from the older hire gear you've been sailing.
But, if you have greater than 14 knots of wind fairly often, then you probably want to "stretch" a bit more and get a fairly early planing short board (without any center board or center fin).
So, what windspeed have you seen most often?
One of the larger GO's or Futura's would give you the best "down the road" potential,
but you will need a much larger rig than 5.0 m2 to get any sort of performance in < 20
Are you using the centerboard in the hire boards all the time?
I'm a little puzzled by your description of the boards you've been sailing.
Do you have a brand and model name?
250 cm long x 600 m wide x 245 liters seems a bit wide, so I'm pretty sure you meant
60 cm wide (600 mm) which is about right for a regular "longboard".
Something a whole lot shorter, and a bit wider,with about 135-155 liters sounds about right. You'd need at least a 7.5 m2 rig and an 8.5 would be even better.
Now is the time to spend a bit more on the mast and boom you'll buy.
Go for the highest carbon content you can afford as this will make all your rigs much lighter and more enjoyable to sail.
Give us a little more info to work with (windspeed.....fresh vs salt water... and anything else you can think of) and I'm sure we can come up with some good suggestions.
Hope this helps,