Conventional victory would cheapen the entire thing. If they already made the Reapers so ubah-powerful (as showcased in 1 Reaper vs. entire fleet in ME1), then it would be a mistake to retroactively downgrade them to be 'beatable' in conventional fight. And it would put human race on even higher pedestal, as if it wasn't big of a problem in enough (in both ME and other space operas/sci-fi). Having the solution to defeat the Reapers be a product of multiple races across the millenias AND using the Reapers' own tools softens the trope.
Nope. In mass effect 1, reaper had geth as aid and also they still managed to do suprise attack on citadel fleet even shepard warned them.
Also citadel races has made many technological advances what makes them more powerful than in mass effect 1 time. Even Normandy what had best technology in the time was massively improved in second series and even more in third series.
Actially shepard alone could have destroyed more than 99% of the reaper forces in the batarian system, if she would have just waited few more hours to having reapers arrive that system and getting destroyed by the supernova.
They could do the same trick in the sol system later, destroying most of the reaper fleet.
High emr attack, allied forces basically get 0 losses in initial attack and reapers suffer heavily, even majority of the reaper forces are in earth.
That means that same alliance fleet could decimate any other reaper consentrations without breaking a sweat. Actually alone with the leviathan aid they could destroy rest of the galaxys reapers.