To add: You are probably trying to use too much rail in your turn--common short boarder new to long boarding mistake. Right on takeoff the board is not planing, so your inside rail will stick and pull the nose down. Think of using the tail as more of a pivot point, put way more weight back there than you would on a short board on your bottom turn, whip the nose around then shift your weight to set the rail and trim it out.
Also, moving the fin farther back will make it harder to pivot the board. So, take out the thruster side fins and put a larger 8-9" slotted fin all the way forward in the fin box. I personally have never liked a thruster setup on long boards.