My former mentor was obsessed with this question at first saying there's no way to prove the anisotropy of light (one-way vs reflected 2-way) until Don Lincoln responded to just slowly separate two sync'd atomic clocks and fire light from one to the other and measure the delay. The problem is moving clocks apart causes them to unsync but this is not a problem for a single clock that measures its own reflected beam. So moving them slowly apart introduces an error which can be accounted for using relativity.
Einstein's clock sync method uses light pulses to sync clocks and then you use those light sync'd clocks to measure the speed of light. The result is dependent on the assumption so it's a circular argument and is deemed untrustworthy.
In Einstein's day (1905) they didn't even yet know about atoms let alone atomic clocks so his clock sync method was all he had. Today we can depend on the universal accuracy of atomic clocks to free us from his method (which is ingrained into the derivation of the equations of relativity).
My solution is to move the clocks apart at a slow constant velocity and fire light from one to the other at a pre-agreed proper time on each clock without stopping. The light will meet where they started and you could use an inteferometer to measure any discrepancy in the one-way light speed. When both clocks move at constant velocity, they both tick at the same proper time rate. It's a simultaneity (relative to their common starting point) that is independent of distance separation. If you stop the clocks, you invoke the twin paradox and a syncing problem between them. I recently wrote something here that could add further clarification but it's in limbo. I'm not sure if the experts disapprove or if it's still confusing.