It's the second thread I see about that, and I'm a bit confused.
The whole point of Legacy is that movements keys will align the character to the camera, so why can't you use the Standard Movement settings? What is this hybrid movement type y'all dream off and how does it work exactly?

By the way, I want to stress that out, movement keys is what causes your character to change direction, Left-Click has nothing to do with it; the same result can be obtained by not pressing any mouse button, so long as the camera is facing the right direction.
I think auto-running gives off this illusion because you don't press movement keys while doing it. The exceptions being Move Left/right, which actually have their behaviour changed to work like the Standard Type's Turn Left/Right.

All this to say, your suggestion would make more sense rephrased as "Revert all controls to Standard when holding LMB". You could even push it further, and instead have an entry in the keybind setting that lets you temporarily switch movement type while that key is held.