Enhance themselves how? Everything we use is a cheap knockoff of their tech. There is literally no plot you can come up with that will make sense for them because they rely on organics somehow being special for reasons or the Reapers being utter morons doing things the most inefficient way possible.
And let me add there are several mistakes in making the Reapers the main enemy. They are too old, too advanced and thus have nothing reasonable to gain from us. And they're too big to be personal enemies. They sound good on paper but once you try to implement them they start causing serious issues.
The best thing you can do, and even with no explanation it is STILL better than what we got, is come up with a reason that as a galactic species, the Reapers consider an even greater threat to be from outside the galaxy. It is an infinitely huge universe and it leads to something even more potentially frightening than the Reapers maybe in the future. But even if you don't go there, it makes sense. Are the Reapers the strongest species in the absolute universe? No, there's always something bigger. And that's their fear.
Now why harvest? Why is just an awful answer that gets black boxed no matter what you do, thanks to ME2. Why do Reapers need organic goo to make more Reapers? IDUNNO. But nobody apparently cared much in ME2, because KOOL I SHOOT A HUMAN REAPER! Well, and you dropped them from space to start your story off. So...herp...space goo makes more reapers and that's why they need organics. And maybe human goo is especially good goo. But that's it. Don't tell me about fires, and war, and synthetics versus organics, and all that other nonsense.
This isn't my idea for the best story ever. Just a story that was better than we got. And honestly, most people could come up with better ideas than the end of ME3 or mine.