I looked at internet pictures of a Peguot 206 and understand why you are concerned that a Start board or a large Rio might be too large to carry on the roof. The good news is most Starts (maybe all), some Rios, and some Naish Kaluha's (maybe all) have an opening on the bow you can use to help tie them onto your car. Most other boards, especially those which are not designed as beginner boards don't have a bow opening. A rope tied to the bow opening and one around the fin plus ropes or straps going around the board and through the door openings can securely fasten a board to your Peguot 206 with or without a roof rack. A large piece of foam rubber can be use to protect the finish on your roof if you don't have a roof rack.
You said you think the Start L "probably has a low lifespan". I guess you meant you think you would outgrow it too soon. My 2001 Start is big enough (230 ltrs) to extend my windsurfing season by 5-10 sessions each year when I really want to stay dry because both air and water temperatures are cold, such as today when I had 25-30 mph winds with air temperatures of 44 fahrenheit (6.66 celsius).
I hope you find what you are looking for soon.