You overstate it by calling it a theory. I'm sure you just watched the Veritasium video [1] on this very subject. The point wasn't to claim that space has a preferred direction (which would contradict known physics) but highlighting that there is no way to prove the speed of light is uniform, only that the round trip speed is 'c'.

Second, even if the speed of light is unbounded (or has a much higher bound) in one direction, that doesn't give a free ride for objects with mass to travel any faster than they currently do. It would take an incredible amount of energy to accelerate such rocks to near/above the constant 'c' and then you'd need to explain how it returns to normal asteroid speeds before crashing into Earth.

[1] https://www.youtube.com/watch?v=pTn6Ewhb27k

Thank you! Haha yes I watched Veritasium video...

With synchronized atomic clocks, I could measure one-way speed - at least well enough to tell the difference between c, c/2, and infinity.

Watch the linked video. He goes into that issue.

No, I'm not going to watch the linked video to find out whether he's really answering my question or not. Tell me why I can't synchronize two atomic clocks that are right next to each other, then put one in a car and drive 1000 km away. Tell me why that won't allow me to correlate departure and arrival times well enough to answer the question.

You are being rather demanding, but I'll bite anyway.

When a synchronized clock is accelerated, it will experience time dilation as compared to the stationary clock; they will no longer be synchronized. You might think: ah, I'll just correct for that using relativistic calculations. But you'd be wrong again because Einstein explicitly states that everything in his theory assumes that the speed of light is uniform.

Really, the video does explain all this and more.

What is the opposite direction anyways? Opposite to what? The original message? What if the two transceivers are rotating around the sun on opposite sides? What is one direction at one point in time is now another direction at another point. Is there a difference between bouncing a signal off of a mirror back to yourself and sending a signal which then gets a response from the receiving party? Is my point coming across? Because like, are we talking about direction relative to a receiver or direction relative to some fixed body in space?

Such a theory is in conflict with more than a century of searches for anisotropies of spacetime. No anisotropies have ever been found, beginning with Michelson's seminal experiment.

What? How did you arrive at that conclusion?