Might want to try a Lessacher 40 cm Formula Duo Weed Fin.
I've sailed alot of Formula and wider Free Ride boards with sails up to 9.2 m2
with this fin in the shallow waters of the Pamlico Sound between Hatteras Island
and the mainland.
The Formula Duo Weed really does shed the weeds, as it Wolfgang's proprietary
It's also very good for overall speed and upwind ability (compared to most other weed fins),
and the rake angle will give you a much greater "safety factor" in the shallows.
If you do run aground, with the tip of the fin, you slow down much more slowly, giving
you time to unhook/sheet out without getting tossed over (or onto) the front of the board.
Hope this helps,
P.S. I agree with Krister.
I would not add inboard footstraps.
If you really can't quite get planing, the back foot right in front of the rear
footstrap (on the rail) works well, and you can move your foot further onto
the board if necessary.
Once you get the board up on top of the water, you can pump the fin with
your back foot to increase your speed and break the board really free and
onto a full plane.
With sufficient sail size, you will be planing in around 8 knots of wind pretty