I have a similar issue with my STB. Some days he's fine on gravel, other days he is clearly sore. I'm still working out what to do. I'm currently attempting the leave a little more hoof on approach, which is entirely against what I know.... He's moving pretty fantastic lately, frogs are healthy, heels are wide. I guess spring time road riding will show how he's doing! @DelP told me that her...grandfather? used to put a layer of clay over the foot before a race on the tender footed horses. Worth a shot, and you can probably just line his foot with some clay from outside.