In fact, I've actually overlooked something. It might be easier and more practical nowadays, as The Black Hole said, to raise atmo samples but also, using ray traced atmosphere. I just tested with 64 atmo samples and AA=3. It's still a little grainy but much better than it was before in this situation.
Luminosity glow looks much cleaner than a light source glow but you do need high render/GI settings(atmo samples isn't as important as it is when using light sources) to get the best out of it. Experiment.