I would keep the main backstory it adds some depth and allows for multiple surprises.
Adding context -
Primary Races and Minor Races
Primary Races are those that have FTL/Warp Drives
Minor Races were abducted and seeded throughout the this Galaxy by an unknown force, they are pre-FTL/Warp civilizations with minor colonies.
Random Primary Races as secondary populations are those that found their way through wormholes into this Galaxy, but got stuck and had to create colonies on various planets to survive.
Everyone in this Galaxy is STUCK, all wormholes that lead here vanish afterwards.
Players are those that were sent by their government and/or explorers that wanted to create a colony else where. They went through the wormholes, however were stranded afterwards.
Basically, everyone in this Galaxy is from another galaxy and is stranded in this one, so everyone has to make a new life for themselves.
These are actually a Raiding/Reaver/Pirate based society that flocked through the wormhole. They have setup outposts hidden in nebulas all through this galaxy and they send out raiding groups to take what resources they can find.
Each race has a past, they all have back stories, and some of that should be revealed in some starting missions so that people understand the race they picked better and their motivations. Each race will have different started missions revealing their back story, as well as why they came through the wormhole to this new place. Everyone is literally brand new to this galaxy. The reason why they have so many defenses and resources is because they were supported in some fashion to become a primary outpost/colony for future expansion.
More Back Story
In reality, the Andromeda Galaxy is actually a place for observation and research, before an all out invasion of the surrounding Galaxies. An extremely intelligent species known as the Vakoth have come to the conclusion that all space faring races could eventually become a threat to their way of life.
The Vakoth use wormhole technology to travel great distances, instead of FTL or warp technologies. Higher end FTL/Warp engines, as well as special high end technology can actually damage subspace over time, their wormhole technology is dependent on an unaffected subspace layer to bore through from their location to their destination.
Through various studies they have recognized many cultures that were on the verge of Warp/FTL capabilities, as well as those that already have the technology. The Vokoth will study these races in-depth and will destroy them before they can create high end warp factors above warp 9. They have determined all known races within their galactic cluster that have the capability of producing this tech or the possibility that they can/will.
By allowing these races to populate this particular galaxy, they can study their behavior, their tactics, resources, and technology more in-depth to counter each race effectively. Once they have enough data they will systematically annihilate each race from existence, starting with this galaxy.