I am talking less about buffing them up and more about having them fight smart. For example, have them head straight for Citadel when they arrive as UberJJK mentioned above. Because they certainly would be capable of it once their full force arrives, and not doing so immediately in canon was just reapers being idiots for gameplay reasons.
My head canon about that is rather simple: They know thanks to Nazara that the Citadel is the seat of galatic power again, but at the same time they know that it is well protected and the Mass Relay to the Widow Nebula is a major choke point due to the funky stuff that the Photeans are supposed to have put into the clouds.
The Relays can only send so much mass through before even the Reapers risk crashing into each other or other objects near the Relays.
So the plan to get past that is easy: Hit every races homeworld - if the Council as the seat of galatic government is unavailable, you hit the seats behind that government while also shutting off communication.
You do that and everyone starts being concerned about their
own homeworld instead of someone else's, splitting the collective force that the galaxy can bear. Remember how Shepard had to fly/run around trying to convince everyone to get together and beat the Reapers off Earth?
All according to plan. They did it all in order to get access to the Citadel, which they did.
They probably would have been surprised a little bit if your Shepard managed to convince the Quarians not to shoot at the Geth, which meant you had both the Geth and the Quarian fleet, but everything else was pretty much accounted for.
Everything else was going fine for the Reapers, they firmly believe in the whole 'living monument to the existance of a race' schick they have,
all their plans revolve around high reward, minimum risk to themselves.
And they have it down pat thanks to having done this thousands/millions of times.