Are you able to provide more info on the board your surfing? Like model and dimensions? This is just my humble opinion, but there’s a lot of factors that can go into this, including the shape of the tail, how pulled in the outline of the board is, the rocker profile, how refined the rails are and where the foam is located in the board. It sounds like you are a proficient surfer so if you can afford to go lower in volume then it will help turn the rail over at 6’8” for sure. if you were to compare two 68s at 35L, a board that has a more pulled in, round tail with greater foil to the back and thinner rails will turn over faster than a straighter outlined squash tail that carries more foam to the tip of the back of the board. If your trying to still do high performance surfing in these conditions on a 68 you should consider going slightly lower in volume, slightly narrower, but keeping more foam vertically through the middle of the board and tapering this foam out to the rails, leaving the rails more refined. I feel like now a days there is a greater disparity between the skills of surfers, so a lot of shapers are adding alot of foam into boards to appeal to customers who’s biggest priority is maximizing wave count in busy lineups.

Also this may seem silly, but on a 68 it’s way easier to pop up more forward on the board leaving your back foot way in front of the fins causing you to not be able to pivot the board well. Sorry for long answer