How would the collector ship have the technology to go between galaxies? I don't think the protheans or collectors could really go between galaxies? *within 200 years
I mean, even the collector ships probably used the mass relays to reduce travel time for traveling hundreds of thousands of light years.
2 million light years.... that's twenty times the span of the Milky Way. The mass relay makes it possible to jump across the Milky Way galaxy in a matter of hours, but without the mass relay, even with FTL drives, it would take hundreds of years. Hundreds of years times twenty. Thousands.
So, in addition to the collector ship, we need a surprise mass relay in the other galaxy. Something as mysterious as the omega relay, except that it winds up in another galaxy, not the center of the Milky Way.