Yeah that’s a good point. Most of the lag would come from within the galaxies, themselves. It tells you more about how far a spiral-in event is embedded within a galaxy and the angle to the event through our own than it does about how far through deep intergalactic space the other galaxy is. The mean free path of a photon in intergalactic space is apparently about 10 billion lightyears.
Then again… photons would be interacting with the quantum fields of intermittent particles (I think?). This may be enough to slow the particles in the effective dielectric of space.