Did Rogue also tell you how the Collective kicked both Equestrian and Draconian butt at the same time on Morn? Granted there were only a few Draconian teams, but that was proof they weren't an immortal fighting force

The truth is, for all the Draconians advanced technology, it seemed people were catching up. Supposedly one of the ships was carrying Dark Energy, which was rare in the Draconian empire. In the Collective, nearly everything is running off dark energy. Add their FTL travel, the use of quantum gates for inter galaxy travel while both the Equestrians and the Collective were moving between galaxies without it.

Light rifles equivalent to human heavy assault rifles? If I remember humans were still using ballistics, while everyone else was using much more powerful weaponry

And their empire was in chaos with the loss of Sanctum, under assault on many different fronts while facing assaults from powerful nations not just from the milky way but from other galaxies (Ylfrish anyone? They kicked Draconian butt too...) For all Rogues talk of tactics, its odd he doesn't consider the effect of losing your entire home galaxy that was acting as a command and control hub. Logistical mayhem, morale loss, broken command structure etc.
Bringing the milky way against them on top of it only shortened the time, their demise would have been ensured, even without the forces of the milky way, the Ylfrish were clearly just as destructive and powerful and presumably actually still had their logistics intact, since they launched a coordinated attack on a Draconian area of resource importance