People need to understand that Shepard preparing wouldn't have made any difference.
The Reapers are a plot device. There's always going to be as many of them as need be for the story to work. Just like the orcs at Helm's Deep. If the galaxy has 50 dreadnoughts, there's going to be 1000 Reaper capital ships. The galaxy has 1,000 dreadnoughts? There's going to be 50,000 capital ships.
You get the idea. The Reapers are going to invade and crush through the galaxy, at least initially, because that's the kind of story this is and that's what they do. Arguing about numbers is pointless, because it's never been about numbers, it's been about narrative.
That's a bit of missing the trees for the forest. Yes, the Reapers are absolutely a plot device. But that's precisely why their numbers and relative strength can be toyed with to decide the plot coupon we have to get to win the plot.
Is it going to be a conventional victory, with the Reapers defeated but surviving? A load bearing boss victory, where the Reapers need to be stalled until the plot coupon fires and kills the Boss Reaper and shuts down the rest, like LoTR? A plot coupon that kills them all? A diplomatic vicotry (I don't have a better name for it - think of Babylon 5 and Sheridan's First One ejecting speech), where the actual power balance is irrelevant to solving the puzzle?
Bioware couldn't really decide how they wanted to beat the Reapers and so their numbers and power became a bit nonsensical. ME1 played with the difference between a conventional victory over Sovereign and a Load Bearing Boss victory (when killing robo-Saren shut it down). ME2 was purely a conventional victory.
This is just part and parcel of MEs problem, which is that there's no actual thematic or conceptual consistency in-between the titles.
So Shepard preparing could absolutely make a difference, depending on how Bioware would have decided to end the plot. Even what form preparing takes would vary based on that idea.