If I would define the one-way speed as non-isotropic, then the transformation formula would be come more complicated, whithout added value:
From what I understand the synchronizations of Reichenbach and Grünbaum are only other conventions of synchronizations. But I don't want a synchronization convention at all. It may have a practical interest, but it's nothing physical.
If Einstein's theory is based on a convention and not on a physical hypothesis it seems to me it has no physical value.