That's a perfectly astute guess, and it sounds more plausible than the original theory behind the EM drive. However photons in a cavity, at these energies, are unlikely to be producing many particles - the effect you're describing is known and can happen (not even with dark matter, photons can directly produce regular matter and antimatter), but we have a good idea of how much is produced and it's not enough to thrust anywhere. (It would be easier just to shoot the photons out the back of the rocket - they're massless and therefore carry momentum with the highest possible energy efficiency.)