Sorry, The Flood Has Not Conquered The Rest Of The Universe
Although this is a fun theory, the sheer size of the universe essentially makes the flood conquering it impossible. At MOST, they would conquer the Milky Ways most immediate neighbors in the Local Group, which is Andromeda and a few smaller galaxies in our "neighborhood."
However, once you start talking about the Virgo Supercluster(which spans 110 MILLION light years across)and beyond, time and space themselves become a wall to ANY civilization, including the Flood. Also, there are many galaxies far older than the Milky Way, so there would probably be civilizations on par or maybe even more advanced than the Forerunners.
In fact, there might even be Precursors or Precursor like beings throughout the Universe as well. The Universe truly is incomprehensible in it's vastness, go watch Cosmos lol.