Interesting take and totally understandable though that’s not quite what happens in the plot:

• The battle for the Citadel at the end of ME1 wasn’t the entire known universe banding together by far. What we see is a couple of ships that happened to be nearby because at that point, most of the universe still doesn’t believe the reapers even exist.
• After the battle, the reapers don’t show up at the Citadel but at the edge of the galaxy. They are still months to years away from eradicating the Alliance. Yes, they have a whole lot of firepower but taking down thousands of planets full of enemies who now know what’s coming takes a lot longer than an attack on a single space station where nobody was prepared for an attack.
• At the beginning of ME2, Shepard doesn’t just wake up at the hospital after the battle you saw in ME1. During a later battle/patrol (?), the Normandy gets ambushed and completely destroyed. Shepard dies and their corpse drifts through space. Cerberus (who were only briefly mentioned in a side quest in ME1) manage to retieve the body and use an experimental technology to bring them back to life (it’s implied that they basically built a Shepard-shaped cyborg who has access to at least some of ME1 Shepard’s memories). The goal is to have a well-respected figurehead who can assemble a squad to take down some critical Reaper infrastructure.